Thermal Flow Meter

ABSTRACT

To obtain a thermal flow meter capable of providing thermal insulation without degrading responsiveness of a temperature detection element. A thermal flow meter  300  of the present invention includes an air flow sensing portion  602  that detects a flow rate by performing heat transfer with a measurement target gas passing through the main passage  124  using a heat transfer surface, a temperature detection element  518  that detects a temperature of the measurement target gas, a circuit package  400  obtained by connecting a processing unit  604  that processes signals of the air flow sensing portion  602  and the temperature detection element  518  to a lead and sealing the processing unit  604  using a first molding resin through a first molding process, and a housing  302  where the circuit package  400  is fixed using a second molding resin through a second molding process, wherein, in the circuit package  400 , a thickness of a temperature detecting portion  452  for sealing the temperature detection element  518  is thinner than that of a package body portion  426  for sealing the processing unit  604.

TECHNICAL FIELD

The present invention relates to a thermal flow meter.

BACKGROUND ART

A thermal flow meter that measure a flow rate of gas is configured toinclude an air flow sensing portion for measuring a flow rate, such thata flow rate of the gas is measured by performing heat transfer betweenthe air flow sensing portion and the gas as a measurement target. Theflow rate measured by the thermal flow meter is widely used as animportant control parameter for various devices. The thermal flow meteris characterized in that a flow rate of gas such as a mass flow rate canbe measured with relatively high accuracy, compared to other types offlow meters.

However, it is desirable to further improve the measurement accuracy ofthe gas flow rate. For example, in a vehicle where an internalcombustion engine is mounted, demands for fuel saving or exhaust gaspurification are high. In order to satisfy such demands, it is desirableto measure the intake air amount which is a main parameter of theinternal combustion engine with high accuracy. The thermal flow meterthat measures the intake air amount guided to the internal combustionengine has a bypass passage that takes a part of the intake air amountand an air flow sensing portion arranged in the bypass passage. The airflow sensing portion measures a state of the measurement target gasflowing through the bypass passage by performing heat transfer with themeasurement target gas and outputs an electric signal representing theintake air amount guided to the internal combustion engine. Thistechnique is discussed, for example, in JP 20011-252796 (PTL 1).

In JP 2009-8619 A (PTL 2), there is discussed a structure of an air flowmeasurement device for measuring a flow rate of a measurement targetfluid passing through the intake pipe. The air flow measurement deviceof PTL 2 has a base that is inserted into a device insertion hole formedin the intake pipe and has a leading end extending in a radial directioninside the intake pipe and an intake temperature detection elementprovided in a leading end of the base and exposed in nearly the centerof the main passage.

CITATION LIST Patent Literature

PTL 1: JP 2011-252796 A

PTL 2: JP 2009-8619 A

SUMMARY OF INVENTION Technical Problem

For example, when the air flow measurement device is installed in anintake pipe of an internal combustion engine of a vehicle, the intakepipe itself has a high temperature due to thermal influence from theinternal combustion engine, and the inside of the intake pipe is cooledby the intake air to a low temperature. Therefore, the heat of theintake pipe may be transmitted to the intake temperature detectionelement through the base and may generate an error. Therefore, it isnecessary to provide thermal insulation between the base and the intaketemperature detection element. In the technique of PTL 2, thermalinsulation with the base is provided to minimize the error, and theintake temperature detection element is exposed inside the main passage.

However, if the intake temperature detection element is exposed insidethe main passage, moisture and the like contained in the measurementtarget gas may generate corrosion. Meanwhile, if the intake temperaturedetection element is sealed with a molding resin in order to preventcorrosion, a heat capacity in the surrounding increases, so thatresponsiveness of the intake temperature detection element may bedegraded.

In view of the aforementioned problems, the present invention aims toprovide a thermal flow meter capable of providing thermal insulationwithout degrading responsiveness of the temperature detection element.

Solution to Problem

To solve the aforementioned problems, the present invention provides athermal flow meter including a circuit package obtained by sealing, witha molding resin, an air flow sensing portion that detects a flow rate byperforming heat transfer with a measurement target gas passing through amain passage using a heat transfer surface, a temperature detectionelement that detects a temperature of the measurement target gas, and aprocessing unit that processes a signal of the air flow sensing portionand the temperature detection element, wherein, in the circuit package,a thickness of the molding resin in a portion for sealing thetemperature detection element is thinner than that of a portion forsealing the processing unit.

Advantageous Effects of Invention

According to the present invention, since a portion for sealing theprocessing unit has a thickness of the molding resin thinner than thatof the molding resin for sealing the temperature detection element, itis possible to reduce a heat capacity in the portion for sealing thetemperature detection element and improve responsiveness of thetemperature detection element. In addition, it is possible to reduce anerror factor of the temperature detection by reducing a heat transferamount from a package body to the temperature detection element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system diagram illustrating an internal combustion enginecontrol system where a thermal flow meter according to an embodiment ofthe invention is used.

FIGS. 2(A) and 2(B) are diagrams illustrating an appearance of thethermal flow meter, in which FIG. 2(A) is a left side view, and FIG.2(B) is a front view.

FIGS. 3(A) and 3(B) are diagrams illustrating an appearance of thethermal flow meter, in which FIG. 3(A) is a right side view, and FIG.3(B) is a rear view.

FIGS. 4(A) and 4(B) are diagrams illustrating an appearance of thethermal flow meter, in which FIG. 4(A) is a plan view, and FIG. 4(B) isa bottom view.

FIGS. 5(A) and 5(B) are diagrams illustrating a housing of the thermalflow meter, in which FIG. 5(A) is a left side view of the housing, andFIG. 5(B) is a front view of the housing.

FIGS. 6(A) and 6(B) are diagrams illustrating a housing of the thermalflow meter, in which FIG. 6(A) is a right side view of the housing, andFIG. 6(B) is a rear view of the housing.

FIG. 7 is a partially enlarged view illustrating a state of a flow pathsurface arranged in the bypass passage.

FIGS. 8(A) to 8(C) are exterior views illustrating a circuit package, inwhich FIG. 8(A) is a left side view, FIG. 8(B) is a front view, and FIG.8(C) is a rear view.

FIG. 9 is a diagram illustrating a state that circuit components aremounted on a frame of the circuit package.

FIG. 10 is an explanatory diagram illustrating a diaphragm and acommunication hole that connects an opening and a gap inside thediaphragm.

FIG. 11 is a diagram illustrating a condition of the circuit packageafter a first resin molding process.

FIG. 12A is a cross-sectional view taken along a line D-D of FIG. 11.

FIG. 12B is a cross-sectional view illustrating another specific exampleof the protrusion.

FIG. 12C is a cross-sectional view illustrating another specific exampleof the protrusion.

FIG. 12D is a cross-sectional view illustrating another specific exampleof the protrusion.

FIG. 13A is a diagram illustrating an overview of a process ofmanufacturing a thermal flow meter and specifically, a process ofproducing a circuit package.

FIG. 13B is a diagram illustrating an overview of a process ofmanufacturing a thermal flow meter and specifically, a process ofproducing a thermal flow meter.

FIG. 14 is a circuit diagram illustrating a flow rate detection circuito the thermal flow meter.

FIG. 15 is an explanatory diagram illustrating an air flow sensingportion of the flow rate detection circuit.

DESCRIPTION OF EMBODIMENTS

Examples for embodying the invention described below (hereinafter,referred to as embodiments) solves various problems desired as apractical product. In particular, the embodiments solve various problemsfor use in a measurement device for measuring an intake air amount of avehicle and exhibit various effects. One of various problems addressedby the following embodiments is described in the “Problems to Be Solvedby the Invention” described above, and one of various effects obtainedby the following embodiments is described in the “Effects of theInvention.” Various problems solved by the following embodiments andvarious effects obtained the following embodiments will be furtherdescribed in the “Description of Embodiments.” Therefore, it would beappreciated that the following embodiments also include other effects orproblems obtained or addressed by the embodiments than those describedin “Problems to Be Solved by the Invention” or “Effects of theInvention.”

In the following embodiments, like reference numerals denote likeelements even when they are inserted in different drawings, and theyhave the same functional effects. The components that have beendescribed in previous paragraphs may not be described by denotingreference numerals and signs in the drawings.

1. Internal Combustion Engine Control System Having Thermal Flow MeterAccording to One Embodiment of the Invention

FIG. 1 is a system diagram illustrating an electronic fuel injectiontype internal combustion engine control system having a thermal flowmeter according to one embodiment of the invention. Based on theoperation of an internal combustion engine 110 having an engine cylinder112 and an engine piston 114, an intake air as a measurement target gas30 is inhaled from an air cleaner 122 and is guided to a combustionchamber of the engine cylinder 112 through a main passage 124 including,for example, an intake body, a throttle body 126, and an intake manifold128. A flow rate of the measurement target gas 30 as an intake airguided to the combustion chamber is measured by a thermal flow meter 300according to the invention. A fuel is supplied from a fuel injectionvalve 152 based on the measured flow rate and is mixed with themeasurement target gas 30 as an intake air, so that the mixed gas isguided to the combustion chamber. It is noted that, in this embodiment,the fuel injection valve 152 is provided in an intake port of theinternal combustion engine, and the fuel injected to the intake port ismixed with the measurement target gas 30 as an intake air to form amixed gas, so that the mixed gas is guided to the combustion chamberthrough an intake valve 116 to generate mechanical energy by burning.

In recent years, in many vehicles, a direct fuel injection method havingexcellent effects in exhaust gas purification or fuel efficiencyimprovement is employed, in which a fuel injection valve 152 isinstalled in a cylinder head of the internal combustion engine, and fuelis directly injected into each combustion chamber from the fuelinjection valve 152. The thermal flow meter 300 may be similarly used ina type in which fuel is directly injected into each combustion chamberas well as a type in which fuel is injected into the intake port of theinternal combustion engine of FIG. 1. A method of measuring controlparameters, including a method of using the thermal flow meter 300, anda method of controlling the internal combustion engine, including a fuelsupply amount or an ignition timing, are similar in basic conceptbetween both types. A representative example of both types, a type inwhich fuel is injected into the intake port is illustrated in FIG. 1.

The fuel and the air guided to the combustion chamber have a fuel/airmixed state and are explosively combusted by spark ignition of theignition plug 154 to generate mechanical energy. The gas aftercombustion is guided to an exhaust pipe from the exhaust valve 118 andis discharged to the outside of the vehicle from the exhaust pipe as anexhaust gas 24. The flow rate of the measurement target gas 30 as anintake air guided to the combustion chamber is controlled by thethrottle valve 132 of which opening level changes in response tomanipulation of an accelerator pedal. The fuel supply amount iscontrolled based on the flow rate of the intake air guided to thecombustion chamber, and a driver controls an opening level of thethrottle valve 132, so that the flow rate of the intake air guided tothe combustion chamber is controlled. As a result, it is possible tocontrol mechanical energy generated by the internal combustion engine.

1.1 Overview of Control of Internal Combustion Engine Control System

The flow rate and the temperature of the measurement target gas 30 as anintake air that is received from the air cleaner 122 and flows throughthe main passage 124 are measured by the thermal flow meter 300, and anelectric signal representing the flow rate and the temperature of theintake air is input to the control device 200 from the thermal flowmeter 300. In addition, an output of the throttle angle sensor 144 thatmeasures an opening level of the throttle valve 132 is input to thecontrol device 200, and an output of a rotation angle sensor 146 isinput to the control device 200 to measure a position or a condition ofthe engine piston 114, the intake valve 116, or the exhaust valve 118 ofthe internal combustion engine and a rotational speed of the internalcombustion engine. In order to measure a mixed ratio state between thefuel amount and the air amount from the condition of exhaust gas 24, anoutput of an oxygen sensor 148 is input to the control device 200.

The control device 200 computes a fuel injection amount or an ignitiontiming based on a flow rate of the intake air as an output of thethermal flow meter 300 and a rotational speed of the internal combustionengine measured from an output of the rotation angle sensor 146. Basedon the computation result of them, a fuel amount supplied from the fuelinjection valve 152 and an ignition timing for igniting the ignitionplug 154 are controlled. In practice, the fuel supply amount or theignition timing is further accurately controlled based on a change ofthe intake temperature or the throttle angle measured by the thermalflow meter 300, a change of the engine rotation speed, and an air-fuelratio state measured by the oxygen sensor 148. In the idle driving stateof the internal combustion engine, the control device 200 furthercontrols the air amount bypassing the throttle valve 132 using an idleair control valve 156 and controls a rotation speed of the internalcombustion engine under the idle driving state.

1.2 Importance of Improvement of Measurement Accuracy of Thermal FlowMeter and Environment for Mounting Thermal Flow Meter

Both the fuel supply amount and the ignition timing as a main controlamount of the internal combustion engine are computed by using an outputof the thermal flow meter 300 as a main parameter. Therefore,improvement of the measurement accuracy, suppression of aging, andimprovement of reliability of the thermal flow meter 300 are importantfor improvement of control accuracy of a vehicle or obtainment ofreliability. In particularly, in recent years, there are a lot ofdemands for fuel saving of vehicles and exhaust gas purification. Inorder to satisfy such demands, it is significantly important to improvethe measurement accuracy of the flow rate of the measurement target gas30 as an intake air measured by the thermal flow meter 300. In addition,it is also important to maintain high reliability of the thermal flowmeter 300.

A vehicle having the thermal flow meter 300 is used under an environmentwhere a temperature change is significant or a coarse weather such as astorm or snow. When a vehicle travels a snowy road, it travels through aroad on which an anti-freezing agent is sprayed. It is preferable thatthe thermal flow meter 300 be designed considering a countermeasure forthe temperature change or a countermeasure for dust or pollutants undersuch a use environment. Furthermore, the thermal flow meter 300 isinstalled under an environment where the internal combustion engine issubjected to vibration. It is also desired to maintain high reliabilityfor vibration.

The thermal flow meter 300 is installed in the intake pipe influenced byheat from the internal combustion engine. For this reason, the heatgenerated from the internal combustion engine is transferred to thethermal flow meter 300 via the intake pipe which is a main passage 124.Since the thermal flow meter 300 measures the flow rate of themeasurement target gas by transferring heat with the measurement targetgas, it is important to suppress influence of the heat from the outsideas much as possible.

The thermal flow meter 300 mounted on a vehicle solves the problemsdescribed in “Problems to Be Solved by the Invention” and provides theeffects described in “Effects of the Invention” as described below. Inaddition, as described below, it solves various problems demanded as aproduct and provides various effects considering various problemsdescribed above. Specific problems or effects solved or provided by thethermal flow meter 300 will be described in the following description ofembodiments.

2. Configuration of Thermal Flow Meter 300

2.1 Exterior Structure of Thermal Flow Meter 300

FIGS. 2(A), 2(B), 3(A), 3(B), 4(A), and 4(B) are diagrams illustratingthe exterior of the thermal flow meter 300, in which FIG. 2(A) is leftside view of the thermal flow meter 300, FIG. 2(B) is a front view, FIG.3(A) is a right side view, FIG. 3(B) is a rear view, FIG. 4(A) is a planview, and FIG. 4(B) is a bottom view. The thermal flow meter 300includes a housing 302, a front cover 303, and a rear cover 304. Thehousing 302 includes a flange 312 for fixing the thermal flow meter 300to an intake body as a main passage 124, an external connector 305having an external terminal 306 for electrical connection to externaldevices, and a measuring portion 310 for measuring a flow rate and thelike. The measuring portion 310 is internally provided with a bypasspassage trench for making a bypass passage. In addition, the measuringportion 310 is internally provided with a circuit package 400 having anair flow sensing portion 602 (refer to FIG. 14) for measuring a flowrate of the measurement target gas 30 flowing through the main passage124 or a temperature detecting portion 452 for measuring a temperatureof the measurement target gas 30 flowing through the main passage 124.

2.2 Effects Based on Exterior Structure of Thermal Flow Meter 300

Since the inlet port 350 of the thermal flow meter 300 is provided inthe leading end side of the measuring portion 310 extending toward thecenter direction of the main passage 124 from the flange 312, the gas inthe vicinity of the center portion distant from the inner wall surfaceinstead of the vicinity of the inner wall surface of the main passage124 may be input to the bypass passage. For this reason, the thermalflow meter 300 can measure a flow rate or a temperature of the airdistant from the inner wall surface of the main passage 124 of thethermal flow meter 300, so that it is possible to suppress a decrease ofthe measurement accuracy caused by influence of heat and the like. Inthe vicinity of the inner wall surface of the main passage 124, thethermal flow meter 300 is easily influenced by the temperature of themain passage 124, so that the temperature of the measurement target gas30 has a different condition from an original temperature of the gas andexhibits a condition different from an average condition of the main gasinside the main passage 124. In particular, if the main passage 124serves as an intake body of the engine, it may be influenced by the heatfrom the engine and remains in a high temperature. For this reason, thegas in the vicinity of the inner wall surface of the main passage 124has a temperature higher than the original temperature of the mainpassage 124 in many cases, so that this degrades the measurementaccuracy.

In the vicinity of the inner wall surface of the main passage 124, afluid resistance increases, and a flow velocity decreases, compared toan average flow velocity in the main passage 124. For this reason, ifthe gas in the vicinity of the inner wall surface of the main passage124 is input to the bypass passage as the measurement target gas 30, adecrease of the flow velocity against the average flow velocity in themain passage 124 may generate a measurement error. In the thermal flowmeter 300 illustrated in FIGS. 2(A), 2(B), 3(A), 3(B), 4(A) and 4(B),since the inlet port 350 is provided in the leading end of the thin andlong measuring portion 310 extending to the center of the main passage124 from the flange 312, it is possible to reduce a measurement errorrelating to a decrease of the flow velocity in the vicinity of the innerwall surface. In the thermal flow meter 300 illustrated in FIGS. 2(A),2(B), 3(A), 3(B), 4(A) and 4(B), in addition to the inlet port 350provided in the leading end of the measuring portion 310 extending tothe center of the main passage 124 from the flange 312, an outlet portof the bypass passage is also provided in the leading end of themeasuring portion 310. Therefore, it is possible to further reduce themeasurement error.

The measuring portion 310 of the thermal flow meter 300 has a shapeextending from the flange 312 to the center direction of the mainpassage 124, and its leading end is provided with the inlet port 350 forinputting a part of the measurement target gas 30 such as an intake airto the bypass passage and the outlet port 352 for returning themeasurement target gas 30 from the bypass passage to the main passage124. While the measuring portion 310 has a shape extending along an axisdirected to the center from the outer wall of the main passage 124, itswidth has a narrow shape as illustrated in FIGS. 2(A) and 3(A). That is,the measuring portion 310 of the thermal flow meter 300 has a frontsurface having an approximately rectangular shape and a side surfacehaving a thin width. As a result, the thermal flow meter 300 can have abypass passage having a sufficient length, and it is possible tosuppress a fluid resistance to a small value for the measurement targetgas 30. For this reason, using the thermal flow meter 300, it ispossible to suppress the fluid resistance to a small value and measurethe flow rate of the measurement target gas 30 with high accuracy.

2.3 Structure of Temperature Detecting Portion 452

The inlet port 343 is positioned in the flange 312 side from the bypasspassage provided in the leading end side of the measuring portion 310and is opened toward an upstream side of the flow of the measurementtarget gas 30 as illustrated in FIGS. 2(A), 2(B), 3(A), and 3(B). Insidethe inlet port 343, a temperature detecting portion 452 is arranged tomeasure a temperature of the measurement target gas 30. In the center ofthe measuring portion 310 where the inlet port 343 is provided, anupstream-side outer wall inside the measuring portion 310 included thehousing 302 is hollowed toward the downstream side, the temperaturedetecting portion 452 is formed to protrude toward the upstream sidefrom the upstream-side outer wall having the hollow shape. In addition,front and rear covers 303 and 304 are provided in both sides of theouter wall having a hollow shape, and the upstream side ends of thefront and rear covers 303 and 304 are formed to protrude toward theupstream side from the outer wall having the hollow shape. For thisreason, the outer wall having the hollow shape and the front and rearcovers 303 and 304 in its both sides form the inlet port 343 forreceiving the measurement target gas 30. The measurement target gas 30received from the inlet port 343 makes contact with the temperaturedetecting portion 452 provided inside the inlet port 343 to measure thetemperature of the temperature detecting portion 452. Furthermore, themeasurement target gas 30 flows along a portion that supports thetemperature detecting portion 452 protruding from the outer wall of thehousing 302 having a hollow shape to the upstream side, and isdischarged to the main passage 124 from a front side outlet port 344 anda rear side outlet port 345 provided in the front and rear covers 303and 304.

2.4 Effects Relating to Temperature Detecting Portion 452

A temperature of the gas flowing to the inlet port 343 from the upstreamside of the direction along the flow of the measurement target gas 30 ismeasured by the temperature detecting portion 452. Furthermore, the gasflows toward a neck portion of the temperature detecting portion 452 forsupporting the temperature detecting portion 452, so that it lowers thetemperature of the portion for supporting the temperature detectingportion 452 to the vicinity of the temperature of the measurement targetgas 30. The temperature of the intake pipe serving as a main passage 124typically increases, and the heat is transferred to the portion forsupporting the temperature detecting portion 452 through theupstream-side outer wall inside the measuring portion 310 from theflange 312 or the thermal insulation 315, so that the temperaturemeasurement accuracy may be influenced. The aforementioned supportportion is cooled as the measurement target gas 30 is measured by thetemperature detecting portion 452 and then flows along the supportportion of the temperature detecting portion 452. Therefore, it ispossible to suppress the heat from being transferred to the portion forsupporting the temperature detecting portion 452 through theupstream-side outer wall inside the measuring portion 310 from theflange 312 or the thermal insulation 315.

In particular, in the support portion of the temperature detectingportion 452, the upstream-side outer wall inside the measuring portion310 has a shape concave to the downstream side (as described below withreference to FIGS. 5(A), 5(B), 6(A), and 6(B)). Therefore, it ispossible to increase a length between the upstream-side outer wallinside the measuring portion 310 and the temperature detecting portion452. While the heat conduction length increases, a length of the coolingportion using the measurement target gas 30 increases. Therefore, it ispossible to also reduce influence of the heat from the flange 312 or thethermal insulation 315. Accordingly, the measurement accuracy isimproved. Since the upstream-side outer wall has a shape concaved to thedownstream side (as described below with reference to FIGS. 5(A), 5(B),6(A), and 6(B)), it is possible to easily fix the circuit package 400(refer to FIGS. 5(A), 5(B), 6(A), and 6(B)) described below.

2.5 Structures and Effects of Upstream-side Side Surface andDownstream-side Side Surface of Measuring Portion 310

An upstream-side protrusion 317 and a downstream-side protrusion 318 areprovided in the upstream-side side surface and the downstream-side sidesurface, respectively, of the measuring portion 310 included in thethermal flow meter 300. The upstream-side protrusion 317 and thedownstream-side protrusion 318 have a shape narrowed along the leadingend to the base, so that it is possible to reduce a fluid resistance ofthe measurement target gas 30 as an intake air flowing through the mainpassage 124. The upstream-side protrusion 317 is provided between thethermal insulation 315 and the inlet port 343. The upstream-sideprotrusion 317 has a large cross section and receives a large heatconduction from the flange 312 or the thermal insulation 315. However,the upstream-side protrusion 317 is cut near the inlet port 343, and alength of the temperature detecting portion 452 from the temperaturedetecting portion 452 of the upstream-side protrusion 317 increases dueto the hollow of the upstream-side outer wall of the housing 302 asdescribed below. For this reason, the heat conduction is suppressed fromthe thermal insulation 315 to the support portion of the temperaturedetecting portion 452.

A gap including the terminal connector 320 and the terminal connector320 described below is formed between the flange 312 or the thermalinsulation 315 and the temperature detecting portion 452. For thisreason, a distance between the flange 312 or the thermal insulation 315and the temperature detecting portion 452 increases, and the front cover303 or the rear cover 304 is provided in this long portion, so that thisportion serves as a cooling surface. Therefore, it is possible to reduceinfluence of the temperature of the wall surface of the main passage 124to the temperature detecting portion 452. In addition, as the distancebetween the flange 312 or the thermal insulation 315 and the temperaturedetecting portion 452 increases, it is possible to guide a part of themeasurement target gas 30 input to the bypass passage to the vicinity ofthe center of the main passage 124. It is possible to suppress adecrease of the measurement accuracy caused by heat transfer from thewall surface of the main passage 124.

As illustrated in FIG. 2(B) or 3(B), both side surfaces of the measuringportion 310 inserted into the main passage 124 have a very narrow shape,and a leading end of the downstream-side protrusion 318 or theupstream-side protrusion 317 has a narrow shape relative to the basewhere the air resistance is reduced. For this reason, it is possible tosuppress an increase of the fluid resistance caused by insertion of thethermal flow meter 300 into the main passage 124. Furthermore, in theportion where the downstream-side protrusion 318 or the upstream-sideprotrusion 317 is provided, the upstream-side protrusion 317 or thedownstream-side protrusion 318 protrudes toward both sides relative toboth side portions of the front cover 303 or the rear cover 304. Sincethe upstream-side protrusion 317 or the downstream-side protrusion 318is formed of a resin molding, they are easily formed in a shape havingan insignificant air resistance. Meanwhile, the front cover 303 or therear cover 304 is shaped to have a wide cooling surface. For thisreason, the thermal flow meter 300 has a reduced air resistance and canbe easily cooled by the measurement target gas flowing through the mainpassage 124.

2.6 Structure and Effects of Flange 312

The flange 312 is provided with a plurality of hollows 314 on its lowersurface which is a portion facing the main passage 124, so as to reducea heat transfer surface with the main passage 124 and make it difficultfor the thermal flow meter 300 to receive influence of the heat. Thescrew hole 313 of the flange 312 is provided to fix the thermal flowmeter 300 to the main passage 124, and a space is formed between asurface facing the main passage 124 around each screw hole 313 and themain passage 124 such that the surface facing the main passage 124around the screw hole 313 recedes from the main passage 124. As aresult, the flange 312 has a structure capable of reducing heat transferfrom the main passage 124 to the thermal flow meter 300 and preventingdegradation of the measurement accuracy caused by heat. Furthermore, inaddition to the heat conduction reduction effect, the hollow 314 canreduce influence of contraction of the resin of the flange 312 duringthe formation of the housing 302.

The thermal insulation 315 is provided in the measuring portion 310 sideof the flange 312. The measuring portion 310 of the thermal flow meter300 is inserted into the inside from an installation hole provided inthe main passage 124 so that the thermal insulation 315 faces the innersurface of the installation hole of the main passage 124. The mainpassage 124 serves as, for example, an intake body, and is maintained ata high temperature in many cases. Conversely, it is conceived that themain passage 124 is maintained at a significantly low temperature whenthe operation is activated in a cold district. If such a high or lowtemperature condition of the main passage 124 affects the temperaturedetecting portion 452 or the measurement of the flow rate describedbelow, the measurement accuracy is degraded. For this reason, aplurality of hollows 316 are provided side by side in the thermalinsulation 315 adjacent to the hole inner surface of the main passage124, and a width of the thermal insulation 315 adjacent to the holeinner surface between the neighboring hollows 316 is significantly thin,which is equal to or smaller than ⅓ of the width of the fluid flowdirection of the hollow 316. As a result, it is possible to reduceinfluence of temperature. In addition, a portion of the thermalinsulation 315 becomes thick. During a resin molding of the housing 302,when the resin is cooled from a high temperature to a low temperatureand is solidified, volumetric shrinkage occurs so that a deformation isgenerated as a stress occurs. By forming the hollow 316 in the thermalinsulation 315, it is possible to more uniformize the volumetricshrinkage and reduce stress concentration.

The measuring portion 310 of the thermal flow meter 300 is inserted intothe inside from the installation hole provided in the main passage 124and is fixed to the main passage 124 using the flange 312 of the thermalflow meter 300 with screws. The thermal flow meter 300 is preferablyfixed to the installation hole provided in the main passage 124 with apredetermined positional relationship. The hollow 314 provided in theflange 312 may be used to determine a positional relationship betweenthe main passage 124 and the thermal flow meter 300. By forming theconvex portion in the main passage 124, it is possible to provide aninsertion relationship between the convex portion and the hollow 314 andfix the thermal flow meter 300 to the main passage 124 in an accurateposition.

2.7 Structures and Effects of External Connector 305 and Flange 312

FIG. 4(A) is a plan view illustrating the thermal flow meter 300. Fourexternal terminal 306 and a calibration terminal 307 are provided insidethe external connector 305. The external terminals 306 include terminalsfor outputting the flow rate and the temperature as a measurement resultof the thermal flow meter 300 and a power terminal for supplying DCpower for operating the thermal flow meter 300. The calibration terminal307 is used to measures the produced thermal flow meter 300 to obtain acalibration value of each thermal flow meter 300 and store thecalibration value in an internal memory of the thermal flow meter 300.In the subsequent measurement operation of the thermal flow meter 300,the calibration data representing the calibration value stored in thememory is used, and the calibration terminal 307 is not used. Therefore,in order to prevent the calibration terminal 307 from hinderingconnection between the external terminals 306 and other externaldevices, the calibration terminal 307 has a shape different from that ofthe external terminal 306. In this embodiment, since the calibrationterminal 307 is shorter than the external terminal 306, the calibrationterminal 307 does not hinder connection even when the connectionterminal connected to the external terminal 306 for connection toexternal devices is inserted into the external connector 305. Inaddition, since a plurality of hollows 308 are provided along theexternal terminal 306 inside the external connector 305, the hollows 308reduce stress concentration caused by shrinkage of resin when the resinas a material of the flange 312 is cooled and solidified.

Since the calibration terminal 307 is provided in addition to theexternal terminal 306 used during the measurement operation of thethermal flow meter 300, it is possible to measure characteristics ofeach thermal flow meter 300 before shipping to obtain a variation of theproduct and store a calibration value for reducing the variation in theinternal memory of the thermal flow meter 300. The calibration terminal307 is formed in a shape different from that of the external terminal306 in order to prevent the calibration terminal 307 from hinderingconnection between the external terminal 306 and external devices afterthe calibration value setting process. In this manner, using the thermalflow meter 300, it is possible to reduce a variation of each thermalflow meter 300 before shipping and improve measurement accuracy.

3. Entire Structure of Housing 302 and its Effects

3.1 Structures and Effects of Bypass Passage and Air Flow SensingPortion

FIGS. 5(A), 5(B), 6(A), and 6(B) illustrate a state of the housing 302when the front and rear covers 303 and 304 are removed from the thermalflow meter 300. FIG. 5(A) is a left side view illustrating the housing302, FIG. 5(B) is a front view illustrating the housing 302, FIG. 6(A)is a right side view illustrating the housing 302, and FIG. 6(B) is arear view illustrating the housing 302. In the housing 302, themeasuring portion 310 extends from the flange 312 to the centerdirection of the main passage 124, and a bypass passage trench forforming the bypass passage is provided in its leading end side. In thisembodiment, the bypass passage trench is provided on both frontside andbackside of the housing 302. FIG. 5(B) illustrates a bypass passagetrench on frontside 332, and FIG. 6(B) illustrates a bypass passagetrench on backside 334. Since an inlet trench 351 for forming the inletport 350 of the bypass passage and an outlet trench 353 for forming theoutlet port 352 are provided in the leading end of the housing 302, thegas distant from the inner wall surface of the main passage 124, thatis, the gas flow through the vicinity of the center of the main passage124 can be received as the measurement target gas 30 from the inlet port350. The gas flowing through the vicinity of the inner wall surface ofthe main passage 124 is influenced by the temperature of the wallsurface of the main passage 124 and has a temperature different from theaverage temperature of the gas flowing through the main passage 124 suchas the intake air in many cases. In addition, the gas flowing throughthe vicinity of the inner wall surface of the main passage 124 has aflow velocity lower than the average flow velocity of the gas flowingthrough the main passage 124 in many cases. Since the thermal flow meter300 according to the embodiment is resistant to such influence, it ispossible to suppress a decrease of the measurement accuracy.

The bypass passage formed by the bypass passage trench on frontside 332or the bypass passage trench on backside 334 described above isconnector to the thermal insulation 315 through the outer wall hollowportion 366, the upstream-side outer wall 335, or the downstream-sideouter wall 336. In addition, the upstream-side outer wall 335 isprovided with the upstream-side protrusion 317, and the downstream-sideouter wall 336 is provided with the downstream-side protrusion 318. Inthis structure, since the thermal flow meter 300 is fixed to the mainpassage 124 using the flange 312, the measuring portion 310 having thecircuit package 400 is fixed to the main passage 124 with highreliability.

In this embodiment, the housing 302 is provided with the bypass passagetrench for forming the bypass passage, and the covers are installed onthe frontside and backside of the housing 302, so that the bypasspassage is formed by the bypass passage trench and the covers. In thisstructure, it is possible to form overall bypass passage trenches as apart of the housing 302 in the resin molding process of the housing 302.In addition, since the dies are provided in both surfaces of the housing302 during formation of the housing 302, it is possible to form both thebypass passage trench on frontside 332 and bypass passage trench onbackside 334 as a part of the housing 302 by using the dies for both thesurfaces. Since the front and rear covers 303 and 304 are provided inboth the surfaces of the housing 302, it is possible to obtain thebypass passages in both surfaces of the housing 302. Since the front andbypass passage trench on frontside 332 and bypass passage trenches onbackside 334 are formed on both the surfaces of the housing 302 usingthe dies, it is possible to form the bypass passage with high accuracyand obtain high productivity.

Referring to FIG. 6(B), a part of the measurement target gas 30 flowingthrough the main passage 124 is input to the inside of the bypasspassage trench on backside 334 from the inlet trench 351 that forms theinlet port 350 and flows through the inside of the bypass passage trenchon backside 334. The bypass passage trench on backside 334 graduallydeepens as the gas flows, and the measurement target gas 30 slowly movesto the front direction as it flows along the trench. In particular, thebypass passage trench on backside 334 is provided with a steep slopeportion 347 that steeply deepens to the upstream portion 342 of thecircuit package 400, so that a part of the air having a light mass movesalong the steep slope portion 347 and then flows through the side of themeasurement surface 430 illustrated in FIG. 5(B) in the upstream portion342 of the circuit package 400. Meanwhile, since a foreign object havinga heavy mass has difficulty in steeply changing its path due to aninertial force, it moves to the side of the backside of measurementsurface 431 illustrated in FIG. 6(B). Then, the foreign object flows tothe measurement surface 430 illustrated in FIG. 5(B) through thedownstream portion 341 of the circuit package 400.

A flow of the measurement target gas 30 in the vicinity of the heattransfer surface exposing portion 436 will be described with referenceto FIGS. 7(A) and 7(B). In the bypass passage trench on frontside 332 ofFIG. 5(B), the air as a measurement target gas 30 moving from theupstream portion 342 of the circuit package 400 to the bypass passagetrench on frontside 332 side flows along the measurement surface 430,and heat transfer is performed with the air flow sensing portion 602 formeasuring a flow rate using the heat transfer surface exposing portion436 provided in the measurement surface 430 in order to measure a flowrate. Both the measurement target gas 30 passing through the measurementsurface 430 or the air flowing from the downstream portion 341 of thecircuit package 400 to the bypass passage trench on frontside 332 flowalong the bypass passage trench on frontside 332 and are discharged fromthe outlet trench 353 for forming the outlet port 352 to the mainpassage 124.

A substance having a heavy mass such as a contaminant mixed in themeasurement target gas 30 has a high inertial force and has difficultyin steeply changing its path to the deep side of the trench along thesurface of the steep slope portion 347 of FIG. 6(B) where a depth of thetrench steeply deepens. For this reason, since a foreign object having aheavy mass moves through the side of the backside of measurement surface431, it is possible to suppress the foreign object from passing throughthe vicinity of the heat transfer surface exposing portion 436. In thisembodiment, since most of foreign objects having a heavy mass other thanthe gas pass through the backside of measurement surface 431 which is arear surface of the measurement surface 430, it is possible to reduceinfluence of contamination caused by a foreign object such as an oilcomponent, carbon, or a contaminant and suppress degradation of themeasurement accuracy. That is, since the path of the measurement targetgas 30 steeply changes along an axis across the flow axis of the mainpassage 124, it is possible to reduce influence of a foreign objectmixed in the measurement target gas 30.

In this embodiment, the flow path including the bypass passage trench onbackside 334 is directed to the flange from the leading end of thehousing 302 along a curved line, and the gas flowing through the bypasspassage in the side closest to the flange flows reversely to the flow ofthe main passage 124, so that the bypass passage in the rear surfaceside as one side of this reverse flow is connected to the bypass passageformed in the front surface side as the other side. As a result, it ispossible to easily fix the heat transfer surface exposing portion 436 ofthe circuit package 400 to the bypass passage and easily receive themeasurement target gas 30 in the position close to the center of themain passage 124.

In this embodiment, there is provided a configuration in which thebypass passage trench on backside 334 and the bypass passage trench onfrontside 332 are penetrated in the front and rear sides of the flowdirection of the measurement surface 430 for measuring the flow rate.Meanwhile, the leading end side of the circuit package 400 is notsupported by the housing 302, but has a cavity portion 382 such that thespace of the upstream portion 342 of the circuit package 400 isconnected to the space of the downstream portion 341 of the circuitpackage 400. Using the configuration penetrating the upstream portion342 of the circuit package 400 and the downstream portion 341 of thecircuit package 400, the bypass passage is formed such that themeasurement target gas 30 moves from the bypass passage trench onbackside 334 formed in one surface of the housing 302 to the bypasspassage trench on frontside 332 formed in the other surface of thehousing 302. In this configuration, it is possible to form the bypasspassage trench on both surfaces of the housing 302 through a singleresin molding process and perform molding with a structure for matchingthe bypass passage trenches on both surfaces.

By clamping both sides of the measurement surface 430 formed in thecircuit package 400 using a mold die to form the housing 302, it ispossible to form the configuration penetrating the upstream portion 342of the circuit package 400 and the downstream portion 341 of the circuitpackage 400, perform resin molding for the housing 302, and embed thecircuit package 400 in the housing 302. Since the housing 302 is formedby inserting the circuit package 400 into the die in this manner, it ispossible to embed the circuit package 400 and the heat transfer surfaceexposing portion 436 to the bypass passage with high accuracy.

In this embodiment, a configuration penetrating the upstream portion 342of the circuit package 400 and the downstream portion 341 of the circuitpackage 400 is provided. However, a configuration penetrating any one ofthe upstream portion 342 and the downstream portion 341 of the circuitpackage 400 may also be provided, and the bypass passage shape thatlinks the bypass passage trench on backside 334 and the bypass passagetrench on frontside 332 may be formed through a single resin moldingprocess.

An inside wall of bypass passage on backside 391 and an outside wall ofbypass passage on backside 392 are provided in both sides of the bypasspassage trench on backside 334, and the inner side surface of the rearcover 304 abuts on the leading end portions of the height direction ofeach of the inside wall of bypass passage on backside 391 and theoutside wall of bypass passage on backside 392, so that the bypasspassage on backside is formed in the housing 302. In addition, an insidewall of bypass passage on frontside 393 and an outside wall of bypasspassage on frontside 394 are provided in both sides of the bypasspassage trench on frontside 332, and the inner side surface of the frontcover 303 abuts on the leading end portions of the height direction ofthe inside wall of bypass passage on frontside 393 and the outside wallof bypass passage on frontside 394, so that the bypass passage onfrontside is formed in the housing 302.

In this embodiment, the measurement target gas 30 dividingly flowsthrough the measurement surface 430 and its rear surface, and the heattransfer surface exposing portion 436 for measuring the flow rate isprovided in one of them. However, the measurement target gas 30 may passthrough only the front surface side of the measurement surface 430instead of dividing the measurement target gas 30 into two passages. Bycurving the bypass passage to follow a second axis across a first axisof the flow direction of the main passage 124, it is possible to gathera foreign object mixed in the measurement target gas 30 to the sidewhere the curve of the second axis is insignificant. By providing themeasurement surface 430 and the heat transfer surface exposing portion436 in the side where the curve of the second axis is significant, it ispossible to reduce influence of a foreign object.

In this embodiment, the measurement surface 430 and the heat transfersurface exposing portion 436 are provided in a link portion between thebypass passage trench on frontside 332 and the bypass passage trench onbackside 334. However, the measurement surface 430 and the heat transfersurface exposing portion 436 may be provided in the bypass passagetrench on frontside 332 or the bypass passage trench on backside 334instead of the link portion between the bypass passage trench onfrontside 332 and the bypass passage trench on backside 334.

An orifice shape is formed in a part of the heat transfer surfaceexposing portion 436 provided in the measurement surface 430 to measurea flow rate (as described below with reference to FIGS. 7(A) and 7(B)),so that the flow velocity increases due to the orifice effect, and themeasurement accuracy is improved. In addition, even if a vortex isgenerated in a flow of the gas in the upstream side of the heat transfersurface exposing portion 436, it is possible to eliminate or reduce thevortex using the orifice and improve measurement accuracy.

Referring to FIGS. 5(A), 5(B), 6(A), and 6(B), an outer wall hollowportion 366 is provided, where the upstream-side outer wall 335 has ahollow shape hollowed to the downstream side in a neck portion of thetemperature detecting portion 452. Due to this outer wall hollow portion366, a distance between the temperature detecting portion 452 and theouter wall hollow portion 366 increases, so that it is possible toreduce influence of the heat transferred via the upstream-side outerwall 335.

Although the circuit package 400 is enveloped by the fixing portion 372for fixation of the circuit package 400, it is possible to increase aforce for fixing the circuit package 400 by further fixing the circuitpackage 400 using the outer wall hollow portion 366. The fixing portion372 envelopes the circuit package 400 along a flow axis of themeasurement target gas 30. Meanwhile, the outer wall hollow portion 366envelops the circuit package 400 across the flow axis of the measurementtarget gas 30. That is, the circuit package 400 is enveloped such thatthe enveloping direction is different with respect to the fixing portion372. Since the circuit package 400 is enveloped along the two differentdirections, the fixing force is increased. Although the outer wallhollow portion 366 is a part of the upstream-side outer wall 335, thecircuit package 400 may be enveloped in a direction different from thatof the fixing portion 372 using the downstream-side outer wall 336instead of the upstream-side outer wall 335 in order to increase thefixing force. For example, a plate portion of the circuit package 400may be enveloped by the downstream-side outer wall 336, or the circuitpackage 400 may be enveloped using a hollow hollowed in the upstreamdirection or a protrusion protruding to the upstream direction providedin the downstream-side outer wall 336. Since the outer wall hollowportion 366 is provided in the upstream-side outer wall 335 to envelopthe circuit package 400, it is possible to provide an effect ofincreasing a thermal resistance between the temperature detectingportion 452 and the upstream-side outer wall 335 in addition to fixationof the circuit package 400.

Since the outer wall hollow portion 366 is provided in a neck portion ofthe temperature detecting portion 452, it is possible to reduceinfluence of the heat transferred from the flange 312 or the thermalinsulation 315 through the upstream-side outer wall 335. Furthermore, atemperature measurement hollow 368 formed by a notch between theupstream-side protrusion 317 and the temperature detecting portion 452is provided. Using the temperature measurement hollow 368, it ispossible to reduce heat transfer to the temperature detecting portion452 through the upstream-side protrusion 317. As a result, it ispossible to improve detection accuracy of the temperature detectingportion 452. In particular, since the upstream-side protrusion 317 has alarge cross section, it easily transfers heat, and a functionality ofthe temperature measurement hollow 368 that suppress heat transferbecomes important.

3.2 Structure and Effects of Air Flow Sensing Portion of Bypass Passage

FIGS. 7(A) and 7(B) are partially enlarged views illustrating a statethat the measurement surface 430 of the circuit package 400 is arrangedinside the bypass passage trench as a cross-sectional view taken alongthe line A-A of FIGS. 6(A) and 6(B). It is noted that FIGS. 7(A) and7(B) are a conceptual diagram omitted and simplified compared to thespecific configuration of FIGS. 5(A), 5(B), 6(A), and 6(B), and detailsmay be slightly modified. The left side of FIGS. 7(A) and 7(B) is aterminated end portion of the bypass passage trench on backside 334, andthe right side is a starting end portion of the bypass passage trench onfrontside 332. Although not illustrated clearly in FIGS. 7(A) and 7(B),penetrating portions are provided in both the left and right sides ofthe circuit package 400 having the measurement surface 430, and thebypass passage trench on backside 334 and the bypass passage trench onfrontside 332 are connected to the left and right sides of the circuitpackage 400 having the measurement surface 430.

The measurement target gas 30, that is received from the inlet port 350and flows through the bypass passage on backside including the bypasspassage trench on backside 334, is guided from the left side of FIG. 7.A part of the measurement target gas 30 flows to a flow path 386 formedby the front surface of the measurement surface 430 of the circuitpackage 400 and the protrusions 356 provided on the front cover 303through a penetrating portion of the upstream portion 342 of the circuitpackage 400, and the remaining measurement target gas 30 flows to a flowpath 387 formed by the backside of measurement surface 431 and the rearcover 304. Then, the measurement target gas 30 flowing through the flowpath 387 moves to the bypass passage on frontside 332 through apenetrating portion of the downstream portion 341 of the circuit package400, joins the measurement target gas 30 flowing through the flow path386, flows through the bypass passage on frontside 332, and is thendischarged to the main passage 124 from the outlet port 352.

Because the bypass passage trench is formed such that the flow path ofthe measurement target gas 30 guided to the flow path 386 through thepenetrating portion of the upstream portion 342 of the circuit package400 from the bypass passage trench on backside 334 is curved wider thanthe flow path guided to the flow path 387, a substance having a heavymass such as a contaminant contained in the measurement target gas 30 isgathered in the flow path 387 being less curved. For this reason, thereis nearly no flow of a foreign object into the flow path 386.

The flow path 386 is structured to form an orifice such that the frontcover 303 is provided successively to the leading end portion of thebypass passage trench on frontside 332, and the protrusion 356 smoothlyprotrudes to the measurement surface 430 side. The measurement surface430 is arranged in one side of the orifice portion of the flow path 386and is provided with the heat transfer surface exposing portion 436 forperforming heat transfer between air flow sensing portion 602 and themeasurement target gas 30. In order to perform measurement of the airflow sensing portion 602 with high accuracy, the measurement target gas30 in the heat transfer surface exposing portion 436 preferably makes alaminar flow having a little vortex. In addition, with the flow velocitybeing faster, the measurement accuracy is more improved. For thisreason, the orifice is formed such that the protrusion 356 provided inthe front cover 303 to face the measurement surface 430 smoothlyprotrudes to the measurement surface 430. This orifice reduces a vortexin the measurement target gas 30 to approximate the flow to a laminarflow. Furthermore, since the flow velocity increases in the orificeportion, and the heat transfer surface exposing portion 436 formeasuring the flow rate is arranged in the orifice portion, themeasurement accuracy of the flow rate is improved.

Since the orifice is formed such that the protrusion 356 protrudes tothe inside of the bypass passage trench to face the heat transfersurface exposing portion 436 provided on the measurement surface 430, itis possible to improve measurement accuracy. The protrusion 356 forforming the orifice is provided on the cover facing the heat transfersurface exposing portion 436 provided on the measurement surface 430. InFIGS. 7(A) and 7(B), since the cover facing the heat transfer surfaceexposing portion 436 provided on the measurement surface 430 is thefront cover 303, the protrusion 356 is provided in the front cover 303.Alternatively, the protrusion 356 may also be provided in the coverfacing the heat transfer surface exposing portion 436 provided on themeasurement surface 430 of the front or rear cover 303 or 304. Dependingon which of the surfaces the measurement surface 430 and the heattransfer surface exposing portion 436 in the circuit package 400 areprovided, the cover that faces the heat transfer surface exposingportion 436 is changed.

Referring to FIGS. 5(A), 5(B), 6(A), and 6(B), a press imprint 442 ofthe die used in the resin molding process for the circuit package 400remains on the backside of measurement surface 431 as a rear surface ofthe heat transfer surface exposing portion 436 provided on themeasurement surface 430. The press imprint 442 does not particularlyhinder the measurement of the flow rate and does not make any problemeven when the press imprint 442 remains. In addition, as describedbelow, it is important to protect a semiconductor diaphragm of the airflow sensing portion 602 when the circuit package 400 is formed throughresin molding. For this reason, pressing of the rear surface of the heattransfer surface exposing portion 436 is important. Furthermore, it isimportant to prevent resin that covers the circuit package 400 fromflowing to the heat transfer surface exposing portion 436. For thisviewpoint, the inflow of the resin is suppressed by enveloping themeasurement surface 430 including the heat transfer surface exposingportion 436 using a die and pressing the rear surface of the heattransfer surface exposing portion 436 using another die. Since thecircuit package 400 is made through transfer molding, a pressure of theresin is high, and pressing from the rear surface of the heat transfersurface exposing portion 436 is important. In addition, since asemiconductor diaphragm is used in the air flow sensing portion 602, aventilation passage for a gap created by the semiconductor diaphragm ispreferably formed. In order to hold and fix a plate and the like forforming the ventilation passage, pressing from the rear surface of theheat transfer surface exposing portion 436 is important.

3.3 Structure for Fixing Circuit Package 400 Using Housing 302 andEffects Thereof

Next, fixation of the circuit package 400 to the housing 302 through aresin molding process will be described again with reference to FIGS.5(A), 5(B), 6(A), and 6(B). The circuit package 400 is arranged in andfixed to the housing 302 such that the measurement surface 430 formed onthe front surface of the circuit package 400 is arranged in apredetermined position of the bypass passage trench for forming thebypass passage, for example, in a link portion between the bypasspassage trench on frontside 332 and the bypass passage trench onbackside 334 in the embodiment of FIGS. 5(A), 5(B), 6(A), and 6(B). Aportion for burying and fixing the circuit package 400 into the housing302 through a resin molding is provided as a fixing portion 372 forburying and fixing the circuit package 400 into the housing 302 in theside slightly closer to the flange 312 from the bypass passage trench.The fixing portion 372 is buried so as to cover the outer circumferenceof the circuit package 400 formed through the first resin moldingprocess.

As illustrated in FIG. 5(B), the circuit package 400 is fixed by thefixing portion 372. The fixing portion 372 includes a circuit package400 using a plane having a height adjoining the front cover 303 and athin portion 376. By making a resin that covers a portion correspondingto the portion 376 thin, it is possible to alleviate contraction causedwhen a temperature of the resin is cooled during formation of the fixingportion 372 and reduce a stress concentration applied to the circuitpackage 400. It is possible to obtain better effects if the rear side ofthe circuit package 400 is formed in the shape described above asillustrated in FIG. 6(B).

The entire surface of the circuit package 400 is not covered by a resinused to form the housing 302, but a portion where the outer wall of thecircuit package 400 is exposed is provided in the flange 312 side of thefixing portion 372. In the embodiment of FIGS. 5(A), 5(B), 6(A), and6(B), the area of a portion exposed from the resin of the housing 302but not enveloped by the housing 302 is larger than the area of aportion enveloped by the resin of the housing 302 out of the outercircumferential surface of the circuit package 400. Furthermore, aportion of the measurement surface 430 of the circuit package 400 isalso exposed from the resin of the housing 302.

Since the circumference of the circuit package 400 is enveloped in thesecond resin molding process for forming the housing 302 by forming apart of the fixing portion 372 that covers the outer wall of the circuitpackage 400 across the entire circumference in a thin band shape, it ispossible to alleviate an excessive stress concentration caused by volumecontraction in the course of solidification of the fixing portion 372.The excessive stress concentration may adversely affect the circuitpackage 400.

In order to more robustly fix the circuit package 400 with a small areaby reducing the area of a portion enveloped by the resin of the housing302 of the outer circumferential surface of the circuit package 400, itis preferable to increase adherence of the circuit package 400 to theouter wall in the fixing portion 372. When a thermoplastic resin is usedto form the housing 302, it is preferable that the thermoplastic resinbe penetrated into fine unevennesses on the outer wall of the circuitpackage 400 while it has low viscosity, and the thermoplastic resin besolidified while it is penetrated into the fine unevennesses of theouter wall. In the resin molding process for forming the housing 302, itis preferable that the inlet port of the thermoplastic resin be providedin the fixing portion 372 and in the vicinity thereof. The viscosity ofthe thermoplastic resin increases as the temperature decreases, so thatit is solidified. Therefore, by flowing the thermoplastic resin having ahigh temperature into the fixing portion 372 or from the vicinitythereof, it is possible to solidify the thermoplastic resin having lowviscosity while it abuts on the outer wall of the circuit package 400.As a result, a temperature decrease of the thermoplastic resin issuppressed, and a low viscosity state is maintained, so that adherencebetween the circuit package 400 and the fixing portion 372 is improved.

By roughening the outer wall surface of the circuit package 400, it ispossible to improve adherence between the circuit package 400 and thefixing portion 372. As a method of roughening the outer wall surface ofthe circuit package 400, there is known a roughening method for formingfine unevennesses on the surface of the circuit package 400, such as asatin-finish treatment, after forming the circuit package 400 throughthe first resin molding process. As the roughening method for formingfine unevennesses on the surface of the circuit package 400, forexample, the roughening may be achieved using sand blasting.Furthermore, the roughening may be achieved through a laser machining.

As another roughening method, an uneven sheet is attached on an innersurface of the die used in the first resin molding process, and theresin is pressed to the die having the sheet on the surface. Even usingthis method, it is possible to form and roughen fine unevennesses on asurface of the circuit package 400. Alternatively, unevennesses may beattached on an inner side of the die for forming the circuit package 400to roughen the surface of the circuit package 400. The surface portionof the circuit package 400 for such roughening is at least a portionwhere the fixing portion 372 is provided. In addition, the adherence isfurther strengthened by roughening a surface portion of the circuitpackage 400 where the outer wall hollow portion 366 is provided.

When the unevenness machining is performed for the surface of thecircuit package 400 using the aforementioned sheet, the depth of thetrench depends on the thickness of the sheet. If the thickness of thesheet increases, the molding of the first resin molding process becomesdifficult, so that the thickness of the sheet has a limitation. If thethickness of the sheet decreases, the depth of the unevenness providedon the sheet in advance has a limitation. For this reason, when theaforementioned sheet is used, it is preferable that the depth of theunevenness between the bottom and the top of the unevenness be set to 10μm or larger and 20 μm or smaller. In the depth smaller than 10 μm, theadherence effect is degraded. The depth larger than 20 μm is difficultto obtain from the aforementioned thickness of the sheet.

In roughening methods other than the aforementioned method of using thesheet, it is preferable to set a thickness of the resin in the firstresin molding process for forming the circuit package 400 to 2 mm orsmaller. For this reason, it is difficult to increase the depth of theunevenness between the bottom and the top of the unevenness to 1 mm orlarger. Conceptually, it is anticipated that adherence between the resinthat covers the circuit package 400 and the resin used to form thehousing 302 increases as the depth of the unevenness between the bottomand the top of the unevenness on the surface of the circuit package 400increases. However, for the reason described above, the depth of theunevenness between the bottom and the top of the unevenness ispreferably set to 1 mm or smaller. That is, if the unevenness having athickness of 10 μm or larger and 1 mm or smaller is provided on thesurface of the circuit package 400, it is preferable to increaseadherence between the resin that covers the circuit package 400 and theresin used to form the housing 302.

A thermal expansion coefficient is different between the thermosettingresin used to form the circuit package 400 and the thermoplastic resinused to form the housing 302 having the fixing portion 372. It ispreferable to prevent an excessive stress generated from this differenceof the thermal expansion coefficient from being applied to the circuitpackage 400.

By forming the fixing portion 372 that envelops the outer circumferenceof the circuit package 400 in a band shape and narrowing the width ofthe band, it is possible to alleviate a stress caused by a difference ofthe thermal expansion coefficient applied to the circuit package 400. Awidth of the band of the fixing portion 372 is set to 10 mm or smaller,and preferably 8 mm or smaller. In this embodiment, since the outer wallhollow portion 366 as a part of the upstream-side outer wall 335 of thehousing 302 as well as the fixing portion 372 envelops the circuitpackage 400 to fix the circuit package 400, it is possible to furtherreduce the width of the band of the fixing portion 372. The circuitpackage 400 can be fixed, for example, if the width is set to 3 mm orlarger.

In order to reduce a stress caused by the difference of the thermalexpansion coefficient, a portion covered by the resin used to form thehousing 302 and an exposed portion without covering are provided on thesurface of the circuit package 400. A plurality of portions where thesurface of the circuit package 400 is exposed from the resin of thehousing 302 are provided, and one of them is to the measurement surface430 having the heat transfer surface exposing portion 436 describedabove. In addition, a portion exposed to a part of the flange 312 siderelative to the fixing portion 372 is provided. Furthermore, the outerwall hollow portion 366 is formed to expose a portion of the upstreamside relative to the outer wall hollow portion 366, and this exposedportion serves as a support portion that supports the temperaturedetecting portion 452. A gap is formed such that a portion of the outersurface of the circuit package 400 in the flange 312 side relative tothe fixing portion 372 surrounds the circuit package 400 across itsouter circumference, particularly, the side facing the flange 312 fromthe downstream side of the circuit package 400 and further across theupstream side of the portion close to the terminal of the circuitpackage 400. Since the gap is formed around the portion where thesurface of the circuit package 400 is exposed, it is possible to reducethe heat amount transferred to the circuit package 400 through theflange 312 from the main passage 124 and suppress degradation ofmeasurement accuracy caused by the heat.

A gap is formed between the circuit package 400 and the flange 312, andthis gap serves as a terminal connector 320. The connection terminal 412of the circuit package 400 and the inner socket of external terminal 361positioned in the housing 302 side of the external terminal 306 areelectrically connected to each other using this terminal connector 320through spot welding, laser welding, and the like. The gap of theterminal connector 320 can suppress heat transfer from the housing 302to the circuit package 400 as described above and is provided as a spacethat can be used to perform a connection work between the connectionterminal 412 of the circuit package 400 and the inner socket of externalterminal 361 of the external terminal 306.

3.4 Formation of Housing 302 Through Second Resin Molding Process andEffects Thereof

In the housing 302 illustrated in FIGS. 5(A), 5(B), 6(A), and 6(B)described above, the circuit package 400 having the air flow sensingportion 602 or the processing unit 604 is manufactured through the firstresin molding process. Then, the housing 302 having, for example, thebypass passage trench on frontside 332 or the bypass passage trench onbackside 334 for forming the bypass passage where the measurement targetgas 30 flows are manufactured through the second resin molding process.Through this second resin molding process, the circuit package 400 isembedded into the resin of the housing 302 and is fixed to the inside ofthe housing 302 through resin molding. As a result, the air flow sensingportion 602 performs heat transfer with the measurement target gas 30,so that a configuration relationship such as a positional relationshipor a directional relationship between the heat transfer surface exposingportion 436 for measuring the flow rate and the bypass passageincluding, for example, the bypass passage trench on frontside 332 orthe bypass passage trench on backside 334 can be maintained withremarkably high accuracy. In addition, it is possible to suppress anerror or deviation generated in each circuit package 400 to a very smallvalue. As a result, it is possible to remarkably improve measurementaccuracy of the circuit package 400. For example, compared to aconventional method in which fixation is performed using an adhesive, itis possible to improve measurement accuracy twice or more. Since thethermal flow meter 300 is typically manufactured in large quantities,the method of using an adhesive along with strict measurement has alimitation in improvement of measurement accuracy. However, if thecircuit package 400 is manufactured through the first resin moldingprocess as in this embodiment, and the bypass passage is then formed inthe second resin molding process for forming the bypass passage wherethe measurement target gas 30 flows while the circuit package 400 andthe bypass passage are fixed, it is possible to remarkably reduce avariation of the measurement accuracy and remarkably improve themeasurement accuracy of each thermal flow meter 300. This similarlyapplies to the embodiment of FIG. 7 as well as the embodiment of FIG. 5or 6.

Further referring to the embodiment of, for example, FIG. 5(A), 5(B),6(A), or 6(B), it is possible to fix the circuit package 400 to thehousing 302 such that a relationship between the bypass passage trenchon frontside 332, the bypass passage trench on backside 334, and theheat transfer surface exposing portion 436 is set to a specificrelationship. As a result, in each of the thermal flow meters 300produced in large quantities, a positional relationship or aconfiguration relationship between the heat transfer surface exposingportion 436 of each circuit package 400 and the bypass passage can beregularly obtained with remarkably high accuracy. Since the bypasspassage trench where the heat transfer surface exposing portion 436 ofthe circuit package 400 is fixed, for example, the bypass passage trenchon frontside 332 and the bypass passage trench on backside 334 can beformed with remarkably high accuracy, a work of forming the bypasspassage in this bypass passage trench is a work for covering both sidesof the housing 302 using the front or rear cover 303 or 304. This workis very simple and is a work process having a few factors of degradingthe measurement accuracy. In addition, the front or rear cover 303 or304 is produced through a resin molding process having high formationaccuracy. Therefore, it is possible to form the bypass passage providedin a specific relationship with the heat transfer surface exposingportion 436 of the circuit package 400 with high accuracy. In thismanner, it is possible to obtain high productivity in addition toimprovement of measurement accuracy.

In comparison, in the related art, the thermal flow meter was producedby fabricating the bypass passage and then bonding the measuring portionto the bypass passage using an adhesive. Such a method of using anadhesive is disadvantageous because a thickness of the adhesive isirregular, and a position or angle of the adhesive is different in eachproduct. For this reason, there was a limitation in improvement of themeasurement accuracy. If this work is performed in mass production, itis further difficult to improve the measurement accuracy.

In the embodiment according to the invention, first, the circuit package400 having the air flow sensing portion 602 is produced through a firstresin molding process, and the circuit package 400 is then fixed throughresin molding while the bypass passage trench for forming the bypasspassage through resin molding is formed through a second resin moldingprocess. As a result, it is possible to form the shape of the bypasspassage trench and fix the air flow sensing portion 602 to the bypasspassage trench with significantly high accuracy.

A portion relating to the measurement of the flow rate, such as the heattransfer surface exposing portion 436 of the air flow sensing portion602 or the measurement surface 430 installed in the heat transfersurface exposing portion 436, is formed on the surface of the circuitpackage 400. Then, the measurement surface 430 and the heat transfersurface exposing portion 436 are exposed from the resin used to form thehousing 302. That is, the heat transfer surface exposing portion 436 andthe measurement surface 430 around the heat transfer surface exposingportion 436 are not covered by the resin used to form the housing 302.The measurement surface 430 formed through the resin molding of thecircuit package 400, the heat transfer surface exposing portion 436, orthe temperature detecting portion 452 is directly used even after theresin molding of the housing 302 to measure a flow rate of the thermalflow meter 300 or a temperature. As a result, the measurement accuracyis improved.

In the embodiment according to the invention, the circuit package 400 isintegratedly formed with the housing 302 to fix the circuit package 400to the housing 302 having the bypass passage. Therefore, it is possibleto fix the circuit package 400 to the housing 302 with a small fixationarea. That is, it is possible to increase the surface area of thecircuit package 400 that does not make contact with the housing 302. Thesurface of the circuit package 400 that does not make contact with thehousing 302 is exposed to, for example, a gap. The heat of the intakepipe is transferred to the housing 302 and is then transferred from thehousing 302 to the circuit package 400. Even if the contact area betweenthe housing 302 and the circuit package 400 is reduced instead ofenveloping the entire surface or most of the surface of the circuitpackage 400 with the housing 302, it is possible to maintain highreliability with high accuracy and fix the circuit package 400 to thehousing 302. For this reason, it is possible to suppress heat transferfrom the housing 302 to the circuit package 400 and suppress a decreaseof the measurement accuracy.

In the embodiment illustrated in FIG. 5(A), 5(B), 6(A), or 6(B), thearea A of the exposed surface of the circuit package 400 can be set tobe equal to or larger than the area B covered by a molding material usedto form the housing 302. In the embodiment, the area A is larger thanthe area B. As a result, it is possible to suppress heat transfer fromthe housing 302 to the circuit package 400. In addition, it is possibleto reduce a stress generated by a difference between a thermal expansioncoefficient of the thermosetting resin used to form the circuit package400 and a thermal expansion coefficient of the thermoplastic resin usedto form the housing 302.

4. Appearance of Circuit Package 400

4.1 Formation of Measurement Surface 430 Having Heat Transfer SurfaceExposing Portion 436

FIGS. 8(A) to 8(C) illustrate an appearance of the circuit package 400formed through the first resin molding process. It is noted that thehatching portion in the appearance of the circuit package 400 indicatesa fixation surface 432 where the circuit package 400 is covered by theresin used in the second resin molding process when the housing 302 isformed through the second resin molding process after the circuitpackage 400 is manufactured through the first resin molding process.FIG. 8(A) is a left side view illustrating the circuit package 400, FIG.8(B) is a front view illustrating the circuit package 400, and the FIG.8(C) is a rear view illustrating the circuit package 400. The circuitpackage 400 is embedded with the air flow sensing portion 602 or theprocessing unit 604 described below, and they are integratedly moldedusing a thermosetting resin. The circuit package 400 includes a packagebody portion 426 having an approximately rectangular planar shape and aprotrusion 424 protruding in a bar shape from an upstream end side ofthe package body portion 426.

On the surface of the circuit package 400 of FIG. 8(B), the measurementsurface 430 serving as a plane for flowing the measurement target gas 30is formed in a shape extending in a flow direction of the measurementtarget gas 30. In this embodiment, the measurement surface 430 has arectangular shape extending in the flow direction of the measurementtarget gas 30. The measurement surface 430 is formed to be thinner thanother portions as illustrated in FIG. 8(A), and a part thereof isprovided with the heat transfer surface exposing portion 436. Theembedded air flow sensing portion 602 performs heat transfer to themeasurement target gas 30 through the heat transfer surface exposingportion 436 to measure a condition of the measurement target gas 30 suchas a flow velocity of the measurement target gas 30 and output anelectric signal representing the flow rate of the main passage 124.

In order to measure a condition of the measurement target gas 30 withhigh accuracy using the embedded air flow sensing portion 602 (refer toFIG. 14), the gas flowing through the vicinity of the heat transfersurface exposing portion 436 preferably makes a laminar flow having alittle vortex. For this reason, it is preferable that there be no heightdifference between the flow path side surface of the heat transfersurface exposing portion 436 and the plane of the measurement surface430 that guides the gas. In this configuration, it is possible tosuppress an irregular stress or a distortion from being applied to theair flow sensing portion 602 while maintaining high flow ratemeasurement accuracy. It is noted that the aforementioned heightdifference may be provided if it does not affect the flow ratemeasurement accuracy.

On the rear surface of the measurement surface 430 of the heat transfersurface exposing portion 436, a press imprint 442 of the die thatsupports an internal substrate or plate during the resin molding of thecircuit package 400 remains as illustrated in FIG. 8(C). The heattransfer surface exposing portion 436 is used to perform heat exchangewith the measurement target gas 30. In order to accurately measure acondition of the measurement target gas 30, it is preferable toappropriately perform heat transfer between the air flow sensing portion602 and the measurement target gas 30. For this reason, it is necessaryto avoid a part of the heat transfer surface exposing portion 436 frombeing covered by the resin in the first resin molding process. Dies areinstalled in both the heat transfer surface exposing portion 436 and thebackside of measurement surface 431 as a rear surface thereof, and aninflow of the resin to the heat transfer surface exposing portion 436 isprevented using this die. A press imprint 442 having a concave shape isformed on the rear surface of the heat transfer surface exposing portion436. In this portion, it is preferable to arrange a device serving asthe air flow sensing portion 602 or the like in the vicinity todischarge the heat generated from the device to the outside as much aspossible. The formed concave portion is less influenced by the resin andeasily discharges heat.

A semiconductor diaphragm corresponding to the heat transfer surfaceexposing portion 436 is formed in an air flow sensing portion (flow ratedetection element) 602 including a semiconductor device. Thesemiconductor diaphragm can be obtained by forming a gap on the rearsurface of the air flow sensing portion 602. If the gap is covered, thesemiconductor diaphragm is deformed, and the measurement accuracy isdegraded due to a change of the pressure inside the gap caused by achange of the temperature. For this reason, in this embodiment, anopening 438 communicating with the gap of the rear surface of thesemiconductor diaphragm is provided on the front surface of the circuitpackage 400, and a link channel for linking the gap of the rear surfaceof the semiconductor diaphragm and the opening 438 is provided insidethe circuit package 400. It is noted that the opening 438 is provided inthe portion not hatched in FIGS. 8(A) to 8(C) in order to prevent theopening 438 from being covered by the resin through the second resinmolding process.

It is necessary to form the opening 438 through the first resin moldingprocess while an inflow of the resin to the portion of the opening 438is suppressed by matching dies to both a portion of the opening 438 anda rear surface thereof and pressing the dies to form the opening 438.Formation of the opening 438 and the link channel that connects the gapon the rear surface of the semiconductor diaphragm and the opening 438will be described below.

4.2 Formation of Temperature Detecting Portion 452 and Protrusion 424and Effects Thereof

The temperature detecting portion 452 provided in the circuit package400 is provided in the leading end of the protrusion 424 extending inthe upstream direction of the measurement target gas 30 in order tosupport the temperature detecting portion 452 and has a function ofdetecting a temperature of the measurement target gas 30. In order todetect a temperature of the measurement target gas 30 with highaccuracy, it is preferable to reduce heat transfer to portions otherthan the measurement target gas 30 as much as possible. The protrusion424 that supports the temperature detecting portion 452 has a shapeprotruding from the package body portion 426 along a wide surface of thepackage body portion 426 and having a leading end thinner than the neckthereof, and is provided with the temperature detecting portion 452 inits leading end portion. Due to such a shape, it is possible to reduceinfluence of the heat from the neck portion of the protrusion 424 to thetemperature detecting portion 452.

After the temperature of the measurement target gas 30 is detected usingthe temperature detecting portion 452, the measurement target gas 30flows along the protrusion 424 to approximate the temperature of theprotrusion 424 to the temperature of the measurement target gas 30. As aresult, it is possible to suppress influence of the temperature of theneck portion of the protrusion 424 to the temperature detecting portion452. In particular, in this embodiment, the temperature detectingportion 452 is thinner in the vicinity of the protrusion 424 having thetemperature detecting portion 452 and is thickened toward the neck ofthe protrusion 424. For this reason, the measurement target gas 30 flowsalong the shape of the protrusion 424 to efficiently cool the protrusion424.

The hatching portion of the neck portion of the protrusion 424 is afixation surface 432 covered by the resin used to form the housing 302in the second resin molding process. A hollow is provided in thehatching portion of the neck portion of the protrusion 424. This showsthat a portion of the hollow shape not covered by the resin of thehousing 302 is provided. If such a portion having a hollow shape notcovered by the resin of the housing 302 in the neck portion of theprotrusion 424 is provided in this manner, it is possible to furthereasily cool the protrusion 424 using the measurement target gas 30.

4.3 Terminal of Circuit Package 400

The circuit package 400 is provided with the connection terminal 412 inorder to supply electric power for operating the embedded air flowsensing portion 602 or the processing unit 604 and output the flow ratemeasurement value or the temperature measurement value. In addition, aterminal 414 is provided in order to inspect whether or not the circuitpackage 400 is appropriately operated, or whether or not an abnormalityis generated in a circuit component or connection thereof. In thisembodiment, the circuit package 400 is formed by performing transfermolding for the air flow sensing portion 602 or the processing unit 604using a thermosetting resin through the first resin molding process. Byperforming the transfer molding, it is possible to improve dimensionalaccuracy of the circuit package 400. However, in the transfer moldingprocess, since a high pressure resin is pressed into the inside of thesealed die where the air flow sensing portion 602 or the processing unit604 is embedded, it is preferable to inspect whether or not there is adefect in the air flow sensing portion 602 or the processing unit 604and such a wiring relationship for the obtained circuit package 400. Inthis embodiment, an inspection terminal 414 is provided, and inspectionis performed for each of the produced circuit packages 400. Since theinspection terminal 414 is not used for measurement, the terminal 414 isnot connected to the inner socket of external terminal 361 as describedabove. In addition, each connection terminal 412 is provided with acurved portion 416 in order to increase a mechanical elastic force. If amechanical elastic force is provided in each connection terminal 412, itis possible to absorb a stress caused by a difference of the thermalexpansion coefficient between the resin of the first resin moldingprocess and the resin of the second resin molding process. That is, eachconnection terminal 412 is influenced by thermal expansion caused by thefirst resin molding process, and the inner socket of external terminal361 connected to each connection terminal 412 are influenced by theresin of the second resin molding process. Therefore, it is possible toabsorb generation of a stress caused by the difference of the resin.

4.4 Fixation of Circuit Package 400 through Second Resin Molding Processand Effects Thereof

In FIGS. 8(A) to 8(C), the hatching portion indicates a fixation surface432 for covering the circuit package 400 using the thermoplastic resinused in the second resin molding process to fix the circuit package 400to the housing 302 in the second resin molding process. As describedabove in relation to FIG. 5(A), 5(B), 6(A), or 6(B), it is important tomaintain high accuracy to provide a specific relationship between themeasurement surface 430, the heat transfer surface exposing portion 436provided in the measurement surface 430, and the shape of the bypasspassage. In the second resin molding process, the bypass passage isformed, and the circuit package 400 is fixed to the housing 302 thatforms the bypass passage. Therefore, it is possible to maintain arelationship between the bypass passage, the measurement surface 430,and the heat transfer surface exposing portion 436 with significantlyhigh accuracy. That is, since the circuit package 400 is fixed to thehousing 302 in the second resin molding process, it is possible toposition and fix the circuit package 400 into the die used to form thehousing 302 having the bypass passage with high accuracy. By injecting athermoplastic resin having a high temperature into this die, the bypasspassage is formed with high accuracy, and the circuit package 400 isfixed with high accuracy.

In this embodiment, the entire surface of the circuit package 400 is nota fixation surface 432 covered by the resin used to form the housing302, but the front surface is exposed to the connection terminal 412side of the circuit package 400. That is, a portion not covered by theresin used to form the housing 302 is provided. In the embodimentillustrated in FIGS. 8(A) to 8(C), out of the front surface of thecircuit package 400, the area that is not enveloped by the resin used toform the housing 302 but is exposed from the resin used to form thehousing 302 is larger than the area of the fixation surface 432enveloped by the resin used to form the housing 302.

A thermal expansion coefficient is different between the thermosettingresin used to form the circuit package 400 and the thermoplastic resinused to form the housing 302 having the fixing portion 372. It ispreferable to prevent a stress caused by this difference of the thermalexpansion coefficient from being applied to the circuit package 400 aslong as possible. By reducing the front surface of the circuit package400 and the fixation surface 432, it is possible to reduce influencebased on the difference of the thermal expansion coefficient. Forexample, it is possible to reduce the fixation surface 432 on the frontsurface of the circuit package 400 by providing a band shape having awidth L.

It is possible to increase a mechanical strength of the protrusion 424by providing the fixation surface 432 in the base of the protrusion 424.It is possible to more robustly fix the circuit package 400 and thehousing 302 to each other by providing, on the front surface of thecircuit package 400, a band-shaped fixation surface along a flow axis ofthe measurement target gas 30 and a fixation surface across the flowaxis of the measurement target gas 30. On the fixation surface 432, aportion surrounding the circuit package 400 in a band shape having awidth L along the measurement surface 430 is the fixation surface alongthe flow axis of the measurement target gas 30 described above, and aportion that covers the base of the protrusion 424 is the fixationsurface across the flow axis of the measurement target gas 30.

5. Mounting of Circuit Components to Circuit Package

5.1 Frame of Circuit Package

FIG. 9 illustrates a frame 512 of the circuit package 400 and a mountingstate of a chip as a circuit component 516 mounted on the frame 512. Itis noted that the dotted line 508 indicates a portion covered by the dieused to mold the circuit package 400. A lead 514 is mechanicallyconnected to the frame 512, and a plate 532 is mounted in the center ofthe frame 512. A chip-like air flow sensing portion 602 and a processingunit 604 as a larger scale integrated (LSI) circuit are mounted on theplate 532. A diaphragm 672 is provided in the air flow sensing portion602, and this corresponds to the heat transfer surface exposing portion436 formed through molding as described above. Each terminal of the airflow sensing portion 602 described below and the processing unit 604 areconnected using a wire 542. Moreover, each terminal of the processingunit 604 and a corresponding lead 514 are connected using a wire 543. Inaddition, the lead 514 positioned between a portion corresponding to theconnection terminal of the circuit package 400 and the plate 532 isconnected to the chip-like circuit component 516 therebetween.

The air flow sensing portion 602 having the diaphragm 672 is arranged inthe most leading end side when the circuit package 400 is obtained inthis manner. The processing unit 604 is arranged in the sidecorresponding to the connection terminal for the air flow sensingportion 602 in an LSI state. In addition, a connection wire 543 isarranged in the terminal side of the processing unit 604. Bysequentially arranging the air flow sensing portion 602, the processingunit 604, the wire 543, the circuit component 516, and the connectionlead 514 in this order from the leading end side of the circuit package400 to the connection terminal, the entire circuit package 400 becomessimple and concise.

A lead is provided to support the plate 532, and this lead is fixed tothe frame 512 using the lead 556 or 558. It is noted that a lead surfacehaving the same area as that of the plate 532 connected to the lead isprovided on the lower surface of the plate 532, and the plate 532 ismounted on the lead surface. This lead surface is grounded. As a result,it is possible to suppress noise by commonly grounding the circuit ofthe air flow sensing portion 602 or the processing unit 604 using thelead surface, so that measurement accuracy of the measurement target gas30 is improved. In addition, a lead 544 is provided in the upstream sideof the flow path from the plate 532, that is, so as to protrude along anaxis directed across the axis of the air flow sensing portion 602, theprocessing unit 604, or the circuit component 516 described above. Atemperature detection element 518, for example, a chip-like thermistoris connected to this lead 544. In addition, a lead 548 is provided inthe vicinity of the processing unit 604 which is a base of theprotrusion, and the leads 544 and 548 are electrically connected using aconnection line (thin line) 546 such as an Au wire. As the leads 548 and544 are directly connected, the heat is transferred to the temperaturedetection element 518 through the leads 548 and 544, so that it may bedifficult to accurately measure a temperature of the measurement targetgas 30. For this reason, by connecting a wire having a smallcross-sectional area and a large thermal resistance, it is possible toincrease a thermal resistance between the leads 548 and 544. As aresult, it is possible to improve temperature measurement accuracy ofthe measurement target gas 30 so as to prevent influence of the heatfrom reaching the temperature detection element 518.

The lead 548 is fixed to the frame 512 through the lead 552 or 554. Aconnection portion between the lead 552 or 554 and the frame 512 isfixed to the frame 512 while it is inclined against the protrudingdirection of the protruding temperature detection element 518, and thedie is also inclined in this area. As the molding resin flows along inthis inclination in the first resin molding process, the molding resinof the first resin molding process smoothly flows to the leading endportion where the temperature detection element 518 is provided, so thatreliability is improved.

In FIG. 9, an arrow 592 indicates a resin injection direction. The leadframe where a circuit component is mounted is covered by the die, and apressed fitting hole 590 for resin injection to the die is provided in acircled position, so that a thermosetting resin is injected into the diealong the direction of the arrow 592. The circuit component 516 or thetemperature detection element 518 and the lead 544 for holding thetemperature detection element 518 are provided along the direction ofthe arrow 592 from the pressed fitting hole 590. In addition, the plate532, the processing unit 604, and the air flow sensing portion 602 arearranged in a direction close to the arrow 592. In this arrangement, theresin smoothly flows in the first resin molding process. In the firstresin molding process, a thermosetting resin is used, so that it isimportant to widen the resin before solidification. For this reason,arrangement of a circuit component of the lead 514 or a wire and arelationship between the pressed fitting hole 590 and the injectiondirection become important.

5.2 Structure for Connecting Gap on Rear Surface of Diaphragm andOpening

FIG. 10 is a diagram illustrating a part of the cross section takenalong a line C-C of FIG. 9 for describing a communication hole 676 thatconnects a gap 674 provided inside the diaphragm 672 and the air flowsensing portion (flow rate detecting element) 602 and the hole 520.

As described below, the air flow sensing portion 602 for measuring theflow rate of the measurement target gas 30 is provided with a diaphragm672, and a gap 674 is provided on the rear surface of the diaphragm 672.Although not illustrated, the diaphragm 672 is provided with an elementfor exchanging heat with the measurement target gas 30 and measuring theflow rate thereby. If the heat is transferred to the elements formed inthe diaphragm 672 through the diaphragm 672 separately from the heatexchange with the measurement target gas 30, it is difficult toaccurately measure the flow rate. For this reason, it is necessary toincrease a thermal resistance of the diaphragm 672 and form thediaphragm 672 as thin as possible.

The air flow sensing portion (flow rate detection element) 602 is buriedand fixed into the first resin of the circuit package 400 formed throughthe first resin molding process such that the heat transfer surface 672of the diaphragm 672 is exposed. The surface of the diaphragm 672 isprovided with the elements (not illustrated) described above (such as aheat generator 608, resistors 652 and 654 as an upstream resistancetemperature detector, and resistors 656 and 658 as a downstreamresistance temperature detector illustrated in FIG. 15). The elementsperform heat transfer with the measurement target gas 30 (notillustrated) through the heat transfer surface 437 on the surface of theelements in the heat transfer surface exposing portion 436 correspondingto the diaphragm 672. The heat transfer surface 437 may be provided onthe surface of each element or may be provided with a thin protectionfilm thereon. It is preferable that heat transfer between the elementsand the measurement target gas 30 be smoothly performed, and direct heattransfers between the elements should be reduced as much as possible.

A portion of the air flow sensing portion (flow rate detection element)602 where the elements are provided is arranged in the heat transfersurface exposing portion 436 of the measurement surface 430, and theheat transfer surface 437 is exposed from the resin used to form themeasurement surface 430. The outer circumference of the air flow sensingportion 602 is covered by the thermosetting resin used in the firstresin molding process for forming the measurement surface 430. If onlythe side face of the air flow sensing portion 602 is covered by thethermosetting resin, and the surface side of the outer circumference ofthe air flow sensing portion 602 (that is, the area around the diaphragm672) is not covered by the thermosetting resin, a stress generated inthe resin used to form the measurement surface 430 is received only bythe side face of the air flow sensing portion 602, so that a distortionmay generated in the diaphragm 672, and characteristics may bedeteriorated. The distortion of the diaphragm 672 is reduced by coveringthe outer circumference portion of the air flow sensing portion 602 withthe thermosetting resin as illustrated in FIG. 10. Meanwhile, if aheight difference between the heat transfer surface 437 and themeasurement surface 430 where the measurement target gas 30 flows islarge, the flow of the measurement target gas 30 is disturbed, so thatmeasurement accuracy is degraded. Therefore, it is preferable that aheight difference W between the heat transfer surface 437 and themeasurement surface 430 where the measurement target gas 30 flows besmall.

The diaphragm 672 is formed thin in order to suppress heat transferbetween each element, and the thin is obtained by forming a gap 674 inthe rear surface of the air flow sensing portion 602. If this gap 674 issealed, a pressure of the gap 674 formed on the rear surface of thediaphragm 672 changes depending on a temperature change. As a pressuredifference between the gap 674 and the surface of the diaphragm 672increases, the diaphragm 672 receives the pressure, and a distortion isgenerated, so that high accuracy measurement becomes difficult. For thisreason, a hole 520 connected to the opening 438 opened to the outside isprovided in the plate 532, and a communication hole 676 that connectsthis hole 520 and the gap 674 is provided. This communication hole 676consists of, for example, a pair of plates including first and secondplates 532 and 536. The first plate 532 is provided with holes 520 and521 and a trench for forming the communication hole 676. Thecommunication hole 676 is formed by covering the trench and the holes520 and 521 with the second plate 536. Using the communication hole 676and the hole 520, the pressures applied to the front and rear surfacesof the diaphragm 672 becomes approximately equal, so that themeasurement accuracy is improved.

As described above, the communication hole 676 can be formed by coveringthe trench and the holes 520 and 521 with the second plate 536.Alternatively, the lead frame may be used as second plate 536. Asdescribed in relation to FIG. 9, the diaphragm 672 and the LSI circuitserving as the processing unit 604 are provided on the plate 532. A leadframe for supporting the plate 532 where the diaphragm 672 and theprocessing unit 604 are mounted is provided thereunder. Therefore, usingthe lead frame, the structure becomes simpler. In addition, the leadframe may be used as a ground electrode. If the lead frame serves as thesecond plate 536, and the communication hole 676 is formed by coveringthe holes 520 and 521 formed in the first plate 532 using the lead frameand covering the trench formed in the first plate 532 using the leadframe in this manner, it is possible to simplify the entire structure.In addition, it is possible to reduce influence of noise from theoutside of the diaphragm 672 and the processing unit 604 because thelead frame serves as a ground electrode.

In the circuit package 400, the press imprint 442 remains on the rearsurface of the circuit package 400 where the heat transfer surfaceexposing portion 436 is formed. In the first resin molding process, inorder to prevent an inflow of the resin to the heat transfer surfaceexposing portion 436, a die such as an insertion die is installed in aportion of the heat transfer surface exposing portion 436, and a die isinstalled in a portion of the press imprint 442 opposite thereto, sothat an inflow of the resin to the heat transfer surface exposingportion 436 is suppressed. By forming a portion of the heat transfersurface exposing portion 436 in this manner, it is possible to measurethe flow rate of the measurement target gas 30 with significantly highaccuracy.

FIG. 11 illustrates a state that the frame of FIG. 9 is molded with athermosetting resin through the first resin molding process and iscovered by the thermosetting resin. Through this molding, themeasurement surface 430 is formed on the front surface of the circuitpackage 400, and the heat transfer surface exposing portion 436 isprovided on the measurement surface 430. In addition, the gap 674 on therear surface of the diaphragm 672 corresponding to the heat transfersurface exposing portion 436 is connected to the opening 438. Thetemperature detecting portion 452 for measuring a temperature of themeasurement target gas 30 is provided in the leading end of theprotrusion 424, and the temperature detection element 518 is embeddedinside. Inside the protrusion 424, in order to suppress heat transfer, alead for extracting the electric signal of the temperature detectionelement 518 is segmented, and a connection line 546 having a largethermal resistance is arranged. As a result, it is possible to suppressheat transfer from the base of the protrusion 424 to the temperaturedetecting portion 452 and influence from the heat.

A slope portion 594 or 596 is formed in the base of the protrusion 424.A flow of the resin in the first resin molding process becomes smooth.In addition, the measurement target gas 30 measured by the temperaturedetecting portion 452 smoothly flows from the protrusion 424 to its baseusing the slope portion 594 or 596 while the temperature detectingportion 452 is installed and operated in a vehicle, so as to cool thebase of the protrusion 424. Therefore, it is possible to reduceinfluence of the heat to the temperature detecting portion 452. Afterthe state of FIG. 11, the lead 514 is separated from each terminal so asto be the connection terminal 412 or the terminal 414.

In the first resin molding process, it is necessary to prevent an inflowof the resin to the heat transfer surface exposing portion 436 or theopening 438. For this reason, in the first resin molding process, aninflow of the resin is suppressed in a position of the heat transfersurface exposing portion 436 or the opening 438. For example, aninsertion die larger than the diaphragm 672 is installed, and a press isinstalled in the rear surface thereof, so that it is pressed from bothsurfaces. As illustrated in FIG. 8(C), the press imprint 442 or 441remains on the rear surface corresponding to the heat transfer surfaceexposing portion 436 or the opening 438 of FIG. 11 or the heat transfersurface exposing portion 436 or the opening 438 of FIG. 8(B).

In FIG. 11, a cutout surface of the lead separated from the frame 512 isexposed from the resin surface, so that moisture or the like may intrudeinto the inside on the cutout surface of the lead during the use. It isimportant to prevent such a problem from the viewpoint of durability orreliability. For example, the lead cutout portion of the slope portion594 or the 596 is covered by the resin through the second resin moldingprocess, and the cutout surface between the lead 552 or 554 and theframe 512 illustrated in FIG. 9 is covered by the resin. As a result, itis possible to prevent erosion of the lead 552 or 554 or intrusion ofwater from the cutout portion. The cutout portion of the lead 552 or 554adjoins an important lead portion that transmits the electric signal ofthe temperature detecting portion 452. Therefore, it is preferable thatthe cutout portion be covered in the second resin molding process.

In the circuit package 400 described above, a thickness of the moldingresin of the protrusion 424 which is a portion for sealing thetemperature detection element 518 is equal to a thickness of the moldingresin of the package body portion 426 which is a portion for sealing theprocessing unit 604 as illustrated in FIG. 12A. However, for example,the thickness of the temperature detecting portion 452 of the protrusion424 may be thinner than the thickness of the base end of the protrusion424 of the thickness of the package body portion 426 as illustrated inFIGS. 12B to 12D.

FIG. 12A is a cross-sectional view taken along a line D-D of FIG. 11,and FIGS. 12B to 12D are cross-sectional views illustrating anotherspecific example of the protrusion 424. FIG. 12B illustrates a thinsheet structure formed in a thin sheet shape such that a stepped portionis provided in each of the front and rear surfaces of a leading end ofthe protrusion 424, and a portion for sealing the temperature detectionelement 518 of the temperature detecting portion 452 is thinned to acertain thickness. FIG. 12C illustrates a tapered structure formed in atapered shape such that the thickness is gradually reduced toward theleading end of the protrusion 424. FIG. 12D illustrates a steppedstructure formed in a stepped shape such that a stepped portion isprovided only in a front surface of the leading end of the protrusion424 to lower a portion of the temperature detection element 518 side ofthe temperature detecting portion 452.

In each structure of FIGS. 12B to 12D, a thickness of the molding resinof the temperature detecting portion 452, more specifically, a thicknessof the molding resin of a portion facing the temperature detectionelement 518 is thinner than a thickness of the package body portion 426.Therefore, it is possible to obtain high responsiveness by reducing aheat capacity around the temperature detection element 518. In addition,it is possible to make it difficult to transfer heat from the base endside of the protrusion 424 and obtain high temperature detectionaccuracy.

In particular, in the thin sheet structure of FIG. 12A, it is possibleto reduce the heat capacity around the temperature detection element 518at maximum compared to the structures of FIGS. 12B and 12C. Therefore,it is possible to obtain high responsiveness. In addition, since thereis no stepped portion in the structure having a tapered cross section ofFIG. 12B, it is possible to prevent a breakdown caused by a stressconcentrated on one point of the protrusion 424. Furthermore, in thestructure having the stepped cross section of FIG. 12C, a rear surfaceof the leading end portion of the protrusion 424 extends coplanarly withthe rear surface of the package body portion 426. Therefore, it ispossible to obtain a higher strength than that of the structure of FIG.12A.

In the protrusion 424, since a lead 544 arranged in the leading end sideof the protrusion 424 to mount the temperature detection element 518 anda lead 548 arranged in the base end side of the protrusion 424 areconnected for sealing by a connection line (for example, a gold (Au)line) 546 having a diameter smaller than that of the lead 544 or 548, itis possible to thermally insulate the heat transferred from the packagebody portion 426 side through the lead and prevent heat transfer to thetemperature detection element 518. Therefore, it is possible to obtainhigh temperature detection accuracy.

6. Process of Producing Thermal Flow Meter 300

6.1 Process of Producing Circuit Package 400

FIGS. 13A and 13B illustrate a process of producing the thermal flowmeter 300, in which FIG. 13A illustrates a process of producing thecircuit package 400 and FIG. 13B illustrates a process of producing athermal flow meter. In FIG. 13A, step 1 shows a process of producing aframe of FIG. 9. This frame is formed, for example, through pressmachining.

In step 2, the plate 532 is first mounted on the frame obtained throughthe step 1, and the air flow sensing portion 602 or the processing unit604 is further mounted on the plate 532. Then, the temperature detectionelement 518 and the circuit component such as a chip capacitor aremounted. In step 2, electrical wiring is performed between circuitcomponents, between the circuit component and the lead, and between theleads. In step 2, the leads 544 and 548 are connected using a connectionline 546 for increasing a thermal resistance. In step 2, the circuitcomponent illustrated in FIG. 9 is mounted on the frame 512, and theelectrical wiring is further performed, so that an electric circuit isformed.

Then, in step 3, through the first resin molding process, molding usinga thermosetting resin is performed. This state is illustrated in FIG.11. In addition, in step 3, each of the connected leads is separatedfrom the frame 512, and the leads are separated from each other, so thatthe circuit package 400 of FIGS. 8(A) to 8(C) is obtained. In thiscircuit package 400, as illustrated in FIGS. 8(A) to 8(C), themeasurement surface 430 or the heat transfer surface exposing portion436 is formed.

In step 4, a visual inspection or an operational inspection is performedfor the obtained circuit package 400. In the first resin molding processof step 3, the electric circuit obtained in step 2 is fixed to theinside of the die, and a high temperature resin is injected into the diewith a high pressure. Therefore, it is preferable to inspect whether ornot there is an abnormality in the electric component or the electricwiring. For this inspection, the terminal 414 is used in addition to theconnection terminal 412 of FIGS. 8(A) to 8(C). It is noted that, becausethe terminal 414 is not used thereafter, it may be cut out from the baseafter this inspection.

6.2 Process of Producing Thermal Flow Meter 300 and Calibration ofCharacteristics

In the process of FIG. 13B, the circuit package 400 produced asillustrated in FIG. 13A and the external terminal 306 are used. In step5, the housing 302 is formed through the second resin molding process.In this housing 302, a bypass passage trench formed of resin, the flange312, or the external connector 305 are formed, and the hatching portionof the circuit package 400 illustrated in FIGS. 8(A) to 8(C) is coveredby the resin in the second resin molding process, so that the circuitpackage 400 is fixed to the housing 302. By combining the production(step 3) of the circuit package 400 through the first resin moldingprocess and the formation of the housing 302 of the thermal flow meter300 through the second resin molding process, the flow rate detectionaccuracy is remarkably improved. In step 6, each inner socket ofexternal terminal 361 is separated. In step 7, the connection terminal412 and the inner socket of external terminal 361 are connected.

The housing 302 is obtained in step 7. Then, in step 8, the front andrear covers 303 and 304 are installed in the housing 302, so that theinside of the housing 302 is sealed with the front and rear covers 303and 304, and the bypass passage for flowing the measurement target gas30 is obtained. In addition, an orifice structure described in relationto FIG. 7 is formed by the protrusion 356 provided in the front or rearcover 303 or 304. It is noted that the front cover 303 is formed throughthe molding of step 10, and the rear cover 304 is formed through themolding of step 11. In addition, the front and rear covers 303 and 304are formed through separate processes using different dies.

In step 9, a characteristic test is performed by guiding the air to thebypass passage in practice. Since a relationship between the bypasspassage and the air flow sensing portion is maintained with highaccuracy as described above, significantly high measurement accuracy isobtained by performing a characteristic calibration through acharacteristic test. In addition, since the molding is performed with apositioning or configuration relationship between the bypass passage andthe air flow sensing portion is determined through the first resinmolding process and the second resin molding process, the characteristicdoes not change much even in a long time use, and high reliability isobtained in addition to the high accuracy.

7. Circuit Configuration of Thermal Flow Meter 300

7.1 Entire Circuit Configuration of Thermal Flow Meter 300

FIG. 14 is a circuit diagram illustrating the flow rate detectioncircuit 601 of the thermal flow meter 300. It is noted that themeasurement circuit relating to the temperature detecting portion 452described in the aforementioned embodiment is also provided in thethermal flow meter 300, but is not illustrated intentionally in FIG. 14.The flow rate detection circuit 601 of the thermal flow meter 300includes the air flow sensing portion 602 having the heat generator 608and the processing unit 604. The processing unit 604 control a heatamount of the heat generator 608 of the air flow sensing portion 602 andoutputs a signal representing the flow rate through the terminal 662based on the output of the air flow sensing portion 602. For thisprocessing, the processing unit 604 includes a central processing unit(hereinafter, referred to as “CPU”) 612, an input circuit 614, an outputcircuit 616, a memory 618 for storing data representing a relationshipbetween the calibration value or the measurement value and the flowrate, and a power circuit 622 for supplying a certain voltage to eachnecessary circuit. The power circuit 622 is supplied with DC power froman external power supply such as a vehicle-mount battery through aterminal 664 and a ground terminal (not illustrated).

The air flow sensing portion 602 is provided with a heat generator 608for heating the measurement target gas 30. A voltage V1 is supplied fromthe power circuit 622 to a collector of a transistor 606 included in acurrent supply circuit of the heat generator 608, and a control signalis applied from the CPU 612 to a base of the transistor 606 through theoutput circuit 616. Based on this control signal, a current is suppliedfrom the transistor 606 to the heat generator 608 through the terminal624. The current amount supplied to the heat generator 608 is controlledby a control signal applied from the CPU 612 to the transistor 606 ofthe current supply circuit of the heat generator 608 through the outputcircuit 616. The processing unit 604 controls the heat amount of theheat generator 608 such that a temperature of the measurement target gas30 increases by a predetermined temperature, for example, 100° C. froman initial temperature by heating using the heat generator 608.

The air flow sensing portion 602 includes a heating control bridge 640for controlling a heat amount of the heat generator 608 and a bridgecircuit of air flow sensing 650 for measuring a flow rate. Apredetermined voltage V3 is supplied to one end of the heating controlbridge 640 from the power circuit 622 through the terminal 626, and theother end of the heating control bridge 640 is connected to the groundterminal 630. In addition, a predetermined voltage V2 is applied to oneend of the bridge circuit of air flow sensing 650 from the power circuit622 through the terminal 625, and the other end of the bridge circuit ofair flow sensing 650 is connected to the ground terminal 630.

The heating control bridge 640 has a resistor 642 which is a resistancetemperature detector having a resistance value changing depending on thetemperature of the heated measurement target gas 30, and the resistors642, 644, 646, and 648 constitute a bridge circuit. A potentialdifference between a node A between the resistors 642 and 646 and a nodeB between the resistors 644 and 648 is input to the input circuit 614through the terminals 627 and 628, and the CPU 612 controls the currentsupplied from the transistor 606 to control the heat amount of the heatgenerator 608 such that the potential difference between the nodes A andB is set to a predetermined value, for example, zero voltage in thisembodiment. The flow rate detection circuit 601 illustrated in FIG. 14heats the measurement target gas 30 using the heat generator 608 suchthat a temperature increases by a predetermined temperature, forexample, 100° C. from an initial temperature of the measurement targetgas 30 at all times. In order to perform this heating control with highaccuracy, resistance values of each resistor of the heating controlbridge 640 are set such that the potential difference between the nodesA and B becomes zero when the temperature of the measurement target gas30 heated by the heat generator 608 increases by a predeterminedtemperature, for example, 100° C. from an initial temperature at alltimes. Therefore, in the flow rate detection circuit 601 of FIG. 14, theCPU 612 controls the electric current supplied to the heat generator 608such that the potential difference between the nodes A and B becomeszero.

The bridge circuit of air flow sensing 650 includes four resistancetemperature detectors of resistors 652, 654, 656, and 658. The fourresistance temperature detectors are arranged along the flow of themeasurement target gas 30 such that the resistors 652 and 654 arearranged in the upstream side in the flow path of the measurement targetgas 30 with respect to the heat generator 608, and the resistors 656 and658 are arranged in the downstream side in the flow path of themeasurement target gas 30 with respect to the heat generator 608. Inaddition, in order to increase the measurement accuracy, the resistors652 and 654 are arranged such that distances to the heat generator 608are approximately equal, and the resistors 656 and 658 are arranged suchthat distances to the heat generator 608 are approximately equal.

A potential difference between a node C between the resistors 652 and656 and a node D between the resistors 654 and 658 is input to the inputcircuit 614 through the terminals 631 and 632. In order to increase themeasurement accuracy, each resistance of the bridge circuit of air flowsensing 650 is set, for example, such that a positional differencebetween the nodes C and D is set to zero while the flow of themeasurement target gas 30 is set to zero. Therefore, while the potentialdifference between the nodes C and D is set to, for example, zero, theCPU 612 outputs, from the terminal 662, an electric signal indicatingthat the flow rate of the main passage 124 is zero based on themeasurement result that the flow rate of the measurement target gas 30is zero.

When the measurement target gas 30 flows along the arrow direction inFIG. 14, the resistor 652 or 654 arranged in the upstream side is cooledby the measurement target gas 30, and the resistors 656 and 658 arrangedin the downstream side of the measurement target gas 30 are heated bythe measurement target gas 30 heated by the heat generator 608, so thatthe temperature of the resistors 656 and 658 increases. For this reason,a potential difference is generated between the nodes C and D of thebridge circuit of air flow sensing 650, and this potential difference isinput to the input circuit 614 through the terminals 631 and 632. TheCPU 612 searches data indicating a relationship between the flow rate ofthe main passage 124 and the aforementioned potential difference storedin the memory 618 based on the potential difference between the nodes Cand D of the bridge circuit of air flow sensing 650 to obtain the flowrate of the main passage 124. An electric signal indicating the flowrate of the main passage 124 obtained in this manner is output throughthe terminal 662. It is noted that, although the terminals 664 and 662illustrated in FIG. 14 are denoted by new reference numerals, they areincluded in the connection terminal 412 of FIG. 5(A), 5(B), 6(A), or6(B) described above.

The memory 618 stores the data indicating a relationship between thepotential difference between the nodes C and D and the flow rate of themain passage 124 and calibration data for reducing a measurement errorsuch as a variation, obtained based on the actual measurement value ofthe gas after production of the circuit package 400. It is noted thatthe actual measurement value of the gas after production of the circuitpackage 400 and the calibration value based thereon are stored in thememory 618 using the external terminal 306 or the calibration terminal307 illustrated in FIGS. 4(A) and 4(B). In this embodiment, the circuitpackage 400 is produced while an arrangement relationship between thebypass passage for flowing the measurement target gas 30 and themeasurement surface 430 or an arrangement relationship between thebypass passage for flowing the measurement target gas 30 and the heattransfer surface exposing portion 436 is maintained with high accuracyand a little variation. Therefore, it is possible to obtain ameasurement result with remarkably high accuracy through calibrationusing the calibration value.

7.2 Configuration of Flow Rate Detection Circuit 601

FIG. 15 is a circuit configuration diagram illustrating a circuitarrangement of the flow rate detection circuit 601 of FIG. 14 describedabove. The flow rate detection circuit 601 is manufactured from asemiconductor chip having a rectangular shape. The measurement targetgas 30 flows along the arrow direction from the left side to the rightside of the flow rate detection circuit 601 illustrated in FIG. 15.

A diaphragm 672 having a rectangular shape with the thin semiconductorchip is formed in the air flow sensing portion (flow rate detectionelement) 602 manufactured from a semiconductor chip. The diaphragm 672is provided with a thin area (that is, the aforementioned heat transfersurface) 603 indicated by the dotted line. The aforementioned gap isformed in the rear surface side of the thin area 603 and communicateswith the opening 438 illustrated in FIG. 8(A) to 8(C) or 5, so that thegas pressure inside the gap depends on the pressure of the gas guidedfrom the opening 438.

By reducing the thickness of the diaphragm 672, the thermal conductivityis lowered, and heat transfer to the resistors 652, 654, 658, and 656provided in the thin area (heat transfer surface) 603 of the diaphragm672 through the diaphragm 672 is suppressed, so that the temperatures ofthe resistors are approximately set through heat transfer with themeasurement target gas 30.

The heat generator 608 is provided in the center of the thin area 603 ofthe diaphragm 672, and the resistor 642 of the heating control bridge640 is provided around the heat generator 608. In addition, theresistors 644, 646, and 648 of the heating control bridge 640 areprovided in the outer side of the thin area 603. The resistors 642, 644,646, and 648 formed in this manner constitute the heating control bridge640.

In addition, the resistors 652 and 654 as upstream resistancetemperature detectors and the resistors 656 and 658 as downstreamresistance temperature detectors are arranged to interpose the heatgenerator 608. The resistors 652 and 654 as upstream resistancetemperature detectors are arranged in the upstream side in the arrowdirection where the measurement target gas 30 flows with respect to theheat generator 608. The resistors 656 and 658 as downstream resistancetemperature detectors are arranged in the downstream side in the arrowdirection where the measurement target gas 30 flows with respect to theheat generator 608. In this manner, the bridge circuit of air flowsensing 650 is formed by the resistors 652, 654, 656, and 658 arrangedin the thin area 603.

Both ends of the heat generator 608 are connected to each of theterminals 624 and 629 illustrated in the lower half of FIG. 15. Here, asillustrated in FIG. 14, the current supplied from the transistor 606 tothe heat generator 608 is applied to the terminal 624, and the terminal629 is grounded.

The resistors 642, 644, 646, and 648 of the heating control bridge 640are connected to each other and are connected to the terminals 626 and630. As illustrated in FIG. 14, the terminal 626 is supplied with apredetermined voltage V3 from the power circuit 622, and the terminal630 is grounded. In addition, the node between the resistors 642 and 646and the node between the resistors 646 and 648 are connected to theterminals 627 and 628, respectively. As illustrated in FIG. 15, theterminal 627 outputs an electric potential of the node A between theresistors 642 and 646, and the terminal 627 outputs an electricpotential of the node B between the resistors 644 and 648. Asillustrated in FIG. 14, the terminal 625 is supplied with apredetermined voltage V2 from the power circuit 622, and the terminal630 is grounded as a ground terminal. In addition, a node between theresistors 654 and 658 is connected to the terminal 631, and the terminal631 outputs an electric potential of the node B of FIG. 14. The nodebetween the resistors 652 and 656 is connected to the terminal 632, andthe terminal 632 outputs an electric potential of the node C illustratedin FIG. 14.

As illustrated in FIG. 15, since the resistor 642 of the heating controlbridge 640 is formed in the vicinity of the heat generator 608, it ispossible to measure the temperature of the gas heated by the heat fromthe heat generator 608 with high accuracy. Meanwhile, since theresistors 644, 646, and 648 of the heating control bridge 640 arearranged distant from the heat generator 608, they are not easilyinfluenced by the heat generated from the heat generator 608. Theresistor 642 is configured to respond sensitively to the temperature ofthe gas heated by the heat generator 608, and the resistors 644, 646,and 648 are configured not to be influenced by the heat generator 608.For this reason, the detection accuracy of the measurement target gas 30using the heating control bridge 640 is high, and the control forheating the measurement target gas 30 by only a predeterminedtemperature from its initial temperature can be performed with highaccuracy.

In this embodiment, a gap is formed in the rear surface side of thediaphragm 672 and communicates with the opening 438 illustrated in FIG.8(A) to 8(C) or 5(A) and 5(B), so that a difference between the pressureof the gap in the rear side of the diaphragm 672 and the pressure in thefront side of the diaphragm 672 does not increase. It is possible tosuppress a distortion of the diaphragm 672 caused by this pressuredifference. This contributes to improvement of the flow rate measurementaccuracy.

As described above, the heat conduction through the diaphragm 672 issuppressed as small as possible by forming the thin area 603 andreducing the thickness of a portion including the thin area 603 in thediaphragm 672. Therefore, while influence of the heat conduction throughthe diaphragm 672 is suppressed, the bridge circuit of air flow sensing650 or the heating control bridge 640 more strongly tends to operatedepending on the temperature of the measurement target gas 30, so thatthe measurement operation is improved. For this reason, high measurementaccuracy is obtained.

While embodiments of the present invention have been describedhereinbefore, the present invention is not limited to the aforementionedembodiments, but various design modifications may be possible withoutdeparting from the spirit and scope of the present invention, asdescribed in the appended claims. For example, the aforementionedembodiments have been described just for easy understanding andillustrative purposes, and they are not necessary limited to all of theconfigurations described above. Furthermore, a part of the configurationof one embodiment may also be substituted with or added to anyconfiguration of other embodiments. Alternatively, addition, deletion,or substitution of other embodiments may be possible for any part of theconfiguration of the embodiment.

INDUSTRIAL AVAILABILITY

The present invention is applicable to a measurement apparatus formeasuring a gas flow rate as described above.

REFERENCE SIGNS LIST

-   300 thermal flow meter-   302 housing-   303 front cover-   304 rear cover-   305 external connector-   306 external terminal-   307 calibration terminal-   310 measuring portion-   320 terminal connector-   332 bypass passage trench on frontside-   334 bypass passage trench on backside-   356 protrusion-   359 resin portion-   361 inner socket of external terminal-   372 fixing portion-   400 circuit package-   412 connection terminal-   414 terminal-   424 protrusion-   430 measurement surface-   432 fixation surface-   436 heat transfer surface exposing portion-   438 opening-   452 temperature detecting portion-   518 temperature detection element-   544 lead-   546 connection line-   590 pressed fitting hole-   594 slope portion-   596 slope portion-   601 flow rate detection circuit-   602 air flow sensing portion-   604 processing unit-   608 heat generator-   640 heating control bridge-   650 bridge circuit of air flow sensing-   672 diaphragm

1. A thermal flow meter comprising a circuit package obtained bysealing, with a molding resin, an air flow sensing portion that detectsa flow rate by performing heat transfer with a measurement target gaspassing through a main passage using a heat transfer surface, atemperature detection element that detects a temperature of themeasurement target gas, and a processing unit that processes a signal ofthe air flow sensing portion and the temperature detection element,wherein, in the circuit package, a thickness of the molding resin in aportion for sealing the temperature detection element is thinner thanthat of a portion for sealing the processing unit.
 2. The thermal flowmeter according to claim 1, wherein the circuit package has a packagebody portion having a planar shape and a protrusion protruding from thepackage body portion along a wide surface of the package body portion, aheat transfer surface exposing portion of the air flow sensing portionis exposed in the package body portion, and the temperature detectionelement is sealed in the protrusion.
 3. The thermal flow meter accordingto claim 2, wherein, in the protrusion, a lead arranged in a leading endside of the protrusion to mount the temperature detection element and alead arranged in a base end side of the protrusion are connected by aconnection line, the connection line having a diameter smaller than thelead.